fix(store): Implement version cache invalidation strategy for JetStreamEventStore

Implements cache invalidation on GetLatestVersion when external writers modify the
JetStream stream. The strategy ensures consistency in multi-store scenarios while
maintaining performance for the single-writer case.

Changes:
- Add cache invalidation logic to GetLatestVersion() that detects stale cache
- Document version cache behavior in JetStreamEventStore struct comment
- Add detailed documentation in CLAUDE.md about cache invalidation strategy
- Add TestJetStreamEventStore_CacheInvalidationOnExternalWrite integration test
- Cache is invalidated by deleting entry, forcing fresh fetch on next check

The implementation follows the acceptance criteria by:
1. Documenting the single-writer assumption in code comments
2. Implementing cache invalidation on GetLatestVersion miss
3. Adding comprehensive test for external write scenarios

Closes #126

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

CLAUDE.md

@@ -122,6 +122,30 @@ if errors.Is(err, aether.ErrVersionConflict) {
 - `ErrVersionConflict` - Sentinel error for version conflicts (use with `errors.Is`)
 - `VersionConflictError` - Detailed error with ActorID, CurrentVersion, and AttemptedVersion
 
+#### Version Cache Invalidation
+
+The JetStreamEventStore maintains an in-memory cache of actor versions to optimize
+repeated version checks during optimistic concurrency control. The cache is designed
+to handle multi-store scenarios where external processes may write to the same
+JetStream stream:
+
+- **Cache hits**: Cached version is returned immediately for performance
+- **Cache misses**: If no cached version exists, JetStream is queried and cached
+- **External writes**: If GetLatestVersion detects a version newer than cached, the cache is invalidated and fresh data is fetched next time
+
+This strategy ensures data consistency even in scenarios with external writers while
+maintaining excellent performance for the single-writer case (where only Aether owns
+the stream).
+
+**Implementation detail**: The cache is invalidated by deleting the entry, forcing
+a fresh fetch from JetStream on the next version check for that actor. This is
+safe because:
+
+1. SaveEvent uses getLatestVersionLocked() which checks JetStream on cache miss
+2. GetLatestVersion always fetches fresh data and detects stale cache entries
+3. Subsequent checks will fetch from JetStream until the cache is repopulated
+
+
 ### Namespace Isolation
 
 Namespaces provide logical boundaries for events and subscriptions.