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>

bitchat/Services/TransportConfig.swift

@@ -50,6 +50,14 @@ enum TransportConfig {
     // Sample interval for per-event debug logs on the inbound hot path.
     static let nostrInboundEventLogInterval: Int = 100
 
+    // Conversation store diagnostics (field observability)
+    // Sample interval for the periodic store-audit "OK" heartbeat line
+    // (first + every Nth audit); violations always log at error level.
+    static let conversationStoreAuditLogInterval: Int = 10
+    // Sample interval for the mirrored-republish debug line in the ID-only
+    // delivery fan-out (first + every Nth republish).
+    static let conversationStoreMirroredRepublishLogInterval: Int = 25
+
     // UI thresholds
     static let uiProcessedNostrEventsCap: Int = 2000
     static let uiChannelInactivityThresholdSeconds: TimeInterval = 9 * 60