findings: adopt the registry in the app module; rename ipv6.* to v6.*

The registry was only used in core-engine. The app still emitted seven codes as
raw strings, so the registry test passed while codes lived outside it - among
them ipv6.broken, which fired on a real network and was in no registry at all.

All seven now take their code, category and severity from a registry entry, so
those three cannot disagree at a call site. Grepping for code = "..." across the
app, engine and probe modules now returns nothing.

ipv6.* -> v6.* is the third instance of the same rule being broken: they
declared Category.IPV6 while the prefix map only knows "v6", so
TestType.category("ipv6.broken") fell through to connectivity and the finding
rolled up under the wrong verdict light. The test-type registry already used v6.

Two severities reconciled rather than assumed:

  connectivity.captive_portal is medium, not high. The registry had guessed
  high; the probe emitting it had always said medium, and the probe was the
  considered value - a captive portal on hotel wifi is what should be there.
  no_internet keeps high, since nothing local fixes that.

  v6.not_offered stays info, and the registry now says why it must. Most
  networks still do not offer IPv6; a warning there lights a yellow verdict on a
  healthy network and teaches people to ignore the light.

Plus a BackHandler: the screen was a plain state variable with nothing tying it
to the back stack, so Back left the app from Settings/History instead of
returning to the run screen.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/build-status.md

@@ -914,3 +914,30 @@ Three tests, one of which uses the exact value observed on the wire.
 Worth recording as a reasoning trap: I had originally raised this as "ULA should probably be kept
 verbatim, like RFC1918, for consistency". The surface analogy pointed the wrong way, and the
 correct answer was the opposite.
+
+### Registry adopted everywhere; v6 findings renamed; Back works (2026-08-01)
+The findings registry was only adopted in `core-engine`. The app module still emitted seven codes
+as raw strings, so the registry test passed while codes existed outside it — including
+`ipv6.broken`, which fired on a real network and was in no registry at all.
+
+All seven now reference registry entries for code, category and severity, so those three cannot
+disagree at a call site. A grep for `code = "…"` across the app, engine and probe modules returns
+nothing.
+
+**`ipv6.*` → `v6.*`.** The third instance of rule 1: they declared `Category.IPV6` while the prefix
+map only knows `v6`, so `TestType.category("ipv6.broken")` fell through to *connectivity* and the
+finding rolled up under the wrong verdict light. The test-type registry already used `v6.`.
+
+Two severities reconciled while merging:
+- `connectivity.captive_portal` is **medium**, not high. The registry had guessed high; the probe
+  that emits it had always said medium, and the probe was the considered value — a captive portal
+  on hotel wifi is what should be there, and logging in clears it. `connectivity.no_internet` is
+  the high one, because nothing the user does locally fixes that.
+- `v6.not_offered` is **info, and the registry says it must stay info**. Most networks still do not
+  offer IPv6 and that is not a fault; a warning here lights a yellow verdict on a healthy network,
+  which teaches people to ignore the light.
+
+Also: a `BackHandler` now returns from Settings/History to the run screen. The screen was a plain
+state variable with nothing connecting it to the back stack, so the system Back gesture left the
+app entirely. Enabled only when there is somewhere to go back to, so Back still exits from the run
+screen.

docs/findings-registry.md

@@ -48,7 +48,7 @@ because it looks authoritative.
 | `connectivity.loss_downstream` | medium | Packets were lost on the way back from the server. | The outbound path: the server received what it was answering. |
 | `connectivity.downstream_blocked` | high | Server-initiated packets never arrive, although round trips work. | Basic reachability: the path forwards replies, just not unsolicited traffic. |
 | `connectivity.downstream_reorder` | low | Downstream packets arrive in a different order than they were sent. | — |
-| `connectivity.captive_portal` | high | A captive portal is intercepting connectivity checks. | — |
+| `connectivity.captive_portal` | medium | A captive portal is intercepting connectivity checks. | — |
 | `connectivity.no_internet` | high | Android's own connectivity checks fail on this network. | — |
 
 ### mtu
@@ -81,6 +81,21 @@ because it looks authoritative.
 | `dns.answer_rewritten` | high | A resolver returned an answer that differs from the authoritative record. | — |
 | `dns.authoritative_unreachable` | medium | The canary zone's authoritative server could not be reached. | — |
 
+### v6
+
+The prefix is `v6.`, matching the test-type registry (`v6.brokenness`, `v6.happy_eyeballs`, …).
+These were `ipv6.*` while declaring `Category.IPV6`; since the prefix map only knows `v6`, they
+rolled up under *connectivity* instead — the third occurrence of rule 1 being broken.
+
+| code | severity | means | rules out |
+|---|---|---|---|
+| `v6.broken` | medium | IPv6 is configured on this network but does not work. | Absence of IPv6: it is provisioned, it simply fails. |
+| `v6.not_offered` | info | This network does not offer IPv6. | — |
+
+`v6.not_offered` is **info and must stay info**. Most networks still do not offer IPv6 and that is
+not a fault; reporting it as a warning lights a yellow verdict on a healthy network, which teaches
+people to ignore the light — the one thing a diagnostic must never do.
+
 ## Adding a finding
 
 1. Add a `FindingSpec` to `FindingRegistry`, and to its `all` list.

echolot-app/app/src/main/kotlin/app/echo_lot/app/MainActivity.kt

@@ -85,6 +85,14 @@ class MainActivity : ComponentActivity() {
                             finish()
                         }
                     }
+                    // Without this, the system Back gesture leaves the activity from Settings or
+                    // History instead of returning to the run screen — the screen is a plain state
+                    // variable, so nothing connects it to the back stack. Registered only when
+                    // there is somewhere to go back to, so Back still exits from the run screen.
+                    androidx.activity.compose.BackHandler(enabled = screen != Screen.RUN) {
+                        screen = Screen.RUN
+                    }
+
                     when (screen) {
                       Screen.SETTINGS -> SettingsScreen(
                         settings = vm.settings,

echolot-app/app/src/main/kotlin/app/echo_lot/app/RunViewModel.kt

@@ -356,8 +356,9 @@ class RunViewModel(app: Application) : AndroidViewModel(app) {
                 when {
                     ev.contains("\"captive_portal\"") -> out.add(
                         Finding(
-                            id = ids.uuid(), code = "connectivity.captive_portal", category = Category.CONNECTIVITY,
+                            id = ids.uuid(), code = FindingRegistry.CAPTIVE_PORTAL.code,
-                            severity = Severity.MEDIUM, confidence = Confidence.HIGH,
+                            category = FindingRegistry.CAPTIVE_PORTAL.category,
+                            severity = FindingRegistry.CAPTIVE_PORTAL.severity, confidence = Confidence.HIGH,
                             title = "Captive portal intercepting connections",
                             description = "The generate_204 check returned a redirect or a page instead of HTTP 204 — a captive portal (login/splash page) is intercepting traffic on this network.",
                             evidenceRefs = listOf(EvidenceRef(t.id)),
@@ -365,8 +366,9 @@ class RunViewModel(app: Application) : AndroidViewModel(app) {
                     )
                     t.status == TestStatus.FAILED -> out.add(
                         Finding(
-                            id = ids.uuid(), code = "connectivity.no_internet", category = Category.CONNECTIVITY,
+                            id = ids.uuid(), code = FindingRegistry.NO_INTERNET.code,
-                            severity = Severity.HIGH, confidence = Confidence.HIGH,
+                            category = FindingRegistry.NO_INTERNET.category,
+                            severity = FindingRegistry.NO_INTERNET.severity, confidence = Confidence.HIGH,
                             title = "No working internet on any network",
                             description = "Android's own generate_204 connectivity checks failed on every active network (no HTTP 204) — this device has no validated internet path.",
                             evidenceRefs = listOf(EvidenceRef(t.id)),
@@ -379,8 +381,9 @@ class RunViewModel(app: Application) : AndroidViewModel(app) {
                 if (ev.contains("MISMATCH")) {
                     out.add(
                         Finding(
-                            id = ids.uuid(), code = "dns.answer_rewritten", category = Category.DNS,
+                            id = ids.uuid(), code = FindingRegistry.DNS_ANSWER_REWRITTEN.code,
-                            severity = Severity.HIGH, confidence = Confidence.HIGH,
+                            category = FindingRegistry.DNS_ANSWER_REWRITTEN.category,
+                            severity = FindingRegistry.DNS_ANSWER_REWRITTEN.severity, confidence = Confidence.HIGH,
                             title = "DNS answers are being rewritten",
                             description = "A canary reference record returned different RDATA than the spec-defined ground truth — something on the path is rewriting DNS answers (interception, filtering, or a middlebox).",
                             evidenceRefs = listOf(EvidenceRef(t.id)),
@@ -389,8 +392,9 @@ class RunViewModel(app: Application) : AndroidViewModel(app) {
                 } else if (ev.contains("\"reached_authoritative\":false")) {
                     out.add(
                         Finding(
-                            id = ids.uuid(), code = "dns.authoritative_unreachable", category = Category.DNS,
+                            id = ids.uuid(), code = FindingRegistry.DNS_AUTHORITATIVE_UNREACHABLE.code,
-                            severity = Severity.MEDIUM, confidence = Confidence.MEDIUM,
+                            category = FindingRegistry.DNS_AUTHORITATIVE_UNREACHABLE.category,
+                            severity = FindingRegistry.DNS_AUTHORITATIVE_UNREACHABLE.severity, confidence = Confidence.MEDIUM,
                             title = "Canary queries don't reach the authoritative server",
                             description = "A per-run nonce name (which cannot be cached) was not answered by the canary server — the resolver is intercepting or failing to reach it.",
                             evidenceRefs = listOf(EvidenceRef(t.id)),
@@ -403,8 +407,9 @@ class RunViewModel(app: Application) : AndroidViewModel(app) {
                 if (ev.contains("address/port-dependent (symmetric NAT")) {
                     out.add(
                         Finding(
-                            id = ids.uuid(), code = "nat.symmetric", category = Category.NAT,
+                            id = ids.uuid(), code = FindingRegistry.NAT_SYMMETRIC.code,
-                            severity = Severity.MEDIUM, confidence = Confidence.HIGH,
+                            category = FindingRegistry.NAT_SYMMETRIC.category,
+                            severity = FindingRegistry.NAT_SYMMETRIC.severity, confidence = Confidence.HIGH,
                             title = "Symmetric NAT — peer-to-peer connections need a relay",
                             description = "The NAT assigns a different external port per destination (address/port-dependent mapping). Direct peer-to-peer connections (calls, games, file transfer) will usually fail and fall back to relays.",
                             evidenceRefs = listOf(EvidenceRef(t.id)),
@@ -421,8 +426,9 @@ class RunViewModel(app: Application) : AndroidViewModel(app) {
                 if (ipv6Provisioned(networks)) {
                     out.add(
                         Finding(
-                            id = ids.uuid(), code = "ipv6.broken", category = Category.IPV6,
+                            id = ids.uuid(), code = FindingRegistry.V6_BROKEN.code,
-                            severity = Severity.MEDIUM, confidence = Confidence.HIGH,
+                            category = FindingRegistry.V6_BROKEN.category,
+                            severity = FindingRegistry.V6_BROKEN.severity, confidence = Confidence.HIGH,
                             title = "IPv6 is configured but not working",
                             description = "This network advertises IPv6 (a global address and/or a default route), but ICMPv6 got no reply on any network. Half-configured IPv6 is worse than none: connections try IPv6 first and stall before falling back.",
                             evidenceRefs = listOf(EvidenceRef(t.id)),
@@ -431,8 +437,9 @@ class RunViewModel(app: Application) : AndroidViewModel(app) {
                 } else {
                     out.add(
                         Finding(
-                            id = ids.uuid(), code = "ipv6.not_offered", category = Category.IPV6,
+                            id = ids.uuid(), code = FindingRegistry.V6_NOT_OFFERED.code,
-                            severity = Severity.INFO, confidence = Confidence.HIGH,
+                            category = FindingRegistry.V6_NOT_OFFERED.category,
+                            severity = FindingRegistry.V6_NOT_OFFERED.severity, confidence = Confidence.HIGH,
                             title = "IPv4-only network (no IPv6 offered)",
                             description = "No IPv6 address or default route was provisioned, so IPv6 tests could not run. This is normal — many networks are still IPv4-only and it is not a fault.",
                             evidenceRefs = listOf(EvidenceRef(t.id)),

echolot-app/core-measurement/src/main/kotlin/app/echo_lot/measurement/FindingRegistry.kt

@@ -89,8 +89,12 @@ object FindingRegistry {
         "Downstream packets arrive in a different order than they were sent.",
     )
 
+    // MEDIUM, not HIGH: a captive portal is a condition to report, not necessarily a fault - on
+    // hotel or cafe wifi it is exactly what should be there, and logging in clears it. NO_INTERNET
+    // is the HIGH one, because nothing the user does locally fixes that. The registry first said
+    // HIGH; the probe emitting it had always said MEDIUM, and the probe was the considered value.
     val CAPTIVE_PORTAL = FindingSpec(
-        "connectivity.captive_portal", Category.CONNECTIVITY, Severity.HIGH,
+        "connectivity.captive_portal", Category.CONNECTIVITY, Severity.MEDIUM,
         "A captive portal is intercepting connectivity checks.",
     )
 
@@ -158,6 +162,31 @@ object FindingRegistry {
         "The canary zone's authoritative server could not be reached.",
     )
 
+    // ---- v6 ----------------------------------------------------------------------------
+    //
+    // Prefix is `v6.`, matching the test-type registry (v6.brokenness, v6.happy_eyeballs, ...).
+    // These were `ipv6.*` while declaring Category.IPV6, but the prefix map only knows "v6", so
+    // they silently rolled up under connectivity: the third instance of a prefix disagreeing with
+    // its category and quietly moving a fault to a different verdict light.
+
+    val V6_BROKEN = FindingSpec(
+        "v6.broken", Category.IPV6, Severity.MEDIUM,
+        "IPv6 is configured on this network but does not work.",
+        rulesOut = "Absence of IPv6: it is provisioned, it simply fails.",
+    )
+
+    /**
+     * INFO deliberately, and it needs to stay that way.
+     *
+     * Most networks still do not offer IPv6, and that is not a fault. Reporting it as a warning
+     * lights a yellow verdict on a perfectly healthy network, which teaches people to ignore the
+     * light — the one thing a diagnostic must never do.
+     */
+    val V6_NOT_OFFERED = FindingSpec(
+        "v6.not_offered", Category.IPV6, Severity.INFO,
+        "This network does not offer IPv6.",
+    )
+
     /** Every registered finding, in declaration order. */
     val all: List<FindingSpec> = listOf(
         UDP_UNREACHABLE, UDP_UNREACHABLE_UPSTREAM, UDP_LOSS, LOSS_UPSTREAM, LOSS_DOWNSTREAM,
@@ -167,6 +196,7 @@ object FindingRegistry {
         NAT_UDP_REBINDING, NAT_SYMMETRIC,
         THROUGHPUT_NO_DELIVERY, THROUGHPUT_BELOW_OFFERED,
         DNS_ANSWER_REWRITTEN, DNS_AUTHORITATIVE_UNREACHABLE,
+        V6_BROKEN, V6_NOT_OFFERED,
     )
 
     private val byCode: Map<String, FindingSpec> = all.associateBy { it.code }