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>

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 }