## What changed
Add `ThreadStoreError::Retryable` for transient failures known to be safe to retry and `ThreadStoreError::OutcomeUnknown` for writes that may already have committed. Recovery remains the operation owner's responsibility; neither category automatically triggers retries.
Preserve existing app-server error mappings and fatal session startup behavior, and report the new categories in shutdown diagnostics without exposing raw error payloads.
## Testing
Add coverage for conservative SQLx error classification, mutation error mappings, session initialization categories, and shutdown diagnostics that omit raw error payloads.
GitOrigin-RevId: 259a9bfaa6ff3cbfa2ee61a01a1f1e744c90f118
