Add field correctness diagnostics: store invariant audit, drop and bounds proofs

ConversationStore.auditInvariants() verifies the per-conversation and
store-level message-ID indexes, caps, timestamp ordering, unread-set
membership, and selection validity - wired to the existing read-receipt
cleanup cadence, loud (.error) on violation, sampled heartbeat when
healthy (~2.8ms per audit at 5k messages, benchmarked and floored).
Router drops log both outcomes (marked failed / skipped by no-downgrade
guard); relay cap evictions, age sweeps, and jittered reconnect delays
log their counts; mirrored republishes get a sampled proof line. 11 new
invariant tests corrupt store state through DEBUG-only hooks since the
single-writer lockdown makes those states unreachable via intents.

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

bitchatTests/Performance/perf-floors.json

@@ -24,7 +24,8 @@
     "formatting.formatMessage": 12261,
     "pipeline.privateIngest": 24848,
     "pipeline.publicIngest": 13102,
-    "store.append": 213201
+    "store.append": 213201,
+    "store.audit": 362
   },
   "floors": {
     "nostrInbound.fresh": 530,
@@ -36,6 +37,7 @@
     "formatting.formatMessage": 3000,
     "pipeline.privateIngest": 6000,
     "pipeline.publicIngest": 3200,
-    "store.append": 53000
+    "store.append": 53000,
+    "store.audit": 90
   }
 }