chore: address review suggestions — docs, comments, test annotation

- Fix ASCII art alignment in CRDT_SYNC_ARCHITECTURE.md (box borders)
- Add star-topology assumption comment on purgeTombstones
- Document internal import rationale in retry_test.dart

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

Author: nickmeinhold

docs/CRDT_SYNC_ARCHITECTURE.md

@@ -15,15 +15,15 @@ The key insight: **Engram's knowledge graph operations are overwhelmingly additi
 │  Device A (phone)                               │
 │  ┌───────────┐  ┌────────────────────────────┐  │
 │  │ Drift/SQL │←→│ CRDT merge layer (HLC)     │──┼──┐
-│  │ (local)   │  │ custom CRDT layer (HLC+LWW)       │  │  │
+│  │ (local)   │  │ custom HLC+LWW             │  │  │
 │  └───────────┘  └────────────────────────────┘  │  │
 └─────────────────────────────────────────────────┘  │
                                                       │ background sync
 ┌─────────────────────────────────────────────────┐  │
 │  Device B (laptop)                              │  │
 │  ┌───────────┐  ┌────────────────────────────┐  │  │
 │  │ Drift/SQL │←→│ CRDT merge layer (HLC)     │──┼──┤
-│  │ (local)   │  │ custom CRDT layer (HLC+LWW)       │  │  │
+│  │ (local)   │  │ custom HLC+LWW             │  │  │
 │  └───────────┘  └────────────────────────────┘  │  │
 └─────────────────────────────────────────────────┘  │
                                                       │

lib/src/providers/crdt_sync_provider.dart

@@ -151,6 +151,8 @@ class CrdtSyncNotifier extends Notifier<CrdtSyncState> {
         ref.invalidate(knowledgeGraphProvider);
 
         // Purge tombstones and sync_log entries we've already consumed.
+        // Safe in star topology (Firestore hub). P2P sync would need
+        // per-peer tombstone tracking — see CRDT_SYNC_ARCHITECTURE.md.
         // Wrapped in try/catch so cleanup failure doesn't fail the sync.
         if (highestPullHlc.isNotEmpty) {
           try {

test/services/retry_test.dart

@@ -1,6 +1,8 @@
 import 'dart:io';
 
 import 'package:anthropic_sdk_dart/anthropic_sdk_dart.dart';
+// HttpMethod is not publicly exported by anthropic_sdk_dart but is required
+// to construct AnthropicClientException in tests. Track upstream issue.
 import 'package:anthropic_sdk_dart/src/generated/client.dart' show HttpMethod;
 import 'package:engram/src/services/elevenlabs_client.dart';
 import 'package:engram/src/services/retry.dart';