findings: v6.broken claimed a cause it had no evidence for

A phone reported "IPv6 is configured but not working" while loading an IPv6-only
site over TCP perfectly well. The finding fired on one signal - ICMPv6 echo
getting no reply - at HIGH confidence. ICMPv6 echo is widely filtered on
networks where IPv6 works, so the two cases are indistinguishable from where the
app stands, and it was picking one.

Same class of error as the multi-homed downstream-loss bug: a confident
measurement of something that was not happening. Now v6.no_icmp_reply, low
severity, medium confidence, naming both explanations. Still reported, because
filtered ICMPv6 breaks Path MTU Discovery - large packets vanish instead of
being reported as too big - which is a fault in its own right.

Corroborating with a real IPv6 connection would separate the two properly, but
needs a target, which runs into the hardcoded-deployment issue already open.

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

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

@@ -479,11 +479,11 @@ class RunViewModel(app: Application) : AndroidViewModel(app) {
                 if (ipv6Provisioned(networks)) {
                     out.add(
                         Finding(
-                            id = ids.uuid(), code = FindingRegistry.V6_BROKEN.code,
-                            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.",
+                            id = ids.uuid(), code = FindingRegistry.V6_NO_ICMP_REPLY.code,
+                            category = FindingRegistry.V6_NO_ICMP_REPLY.category,
+                            severity = FindingRegistry.V6_NO_ICMP_REPLY.severity, confidence = Confidence.MEDIUM,
+                            title = "IPv6 is configured, but ICMPv6 gets no reply",
+                            description = "This network advertises IPv6 (a global address and/or a default route), but ICMPv6 echo got no reply on any network. That has two explanations which look identical from here: IPv6 is broken, or ICMPv6 is filtered while IPv6 itself works. Filtering is common and is a fault in its own right — it breaks Path MTU Discovery, so large packets vanish rather than being reported as too big.",
                             evidenceRefs = listOf(EvidenceRef(t.id)),
                         )
                     )