feat: add message versioning and improve IndexedDB cache cleanup
- Add MESSAGE_VERSION constant for forward compatibility checking
- Version all window.postMessage() calls to detect format incompatibilities
- Improve IndexedDB cleanup to track total cache size across all entries
- Fall back to entry.body.length when bodyChars is missing
- Remove dead code checking typeof readStoredExtraMessages
- Improve error handling in idbRequestToPromise with explicit error object

fix: prevent race condition in route changes with generation token
- Add routeChangeSeq generation token to detect stale async callbacks
- Add debounce (100ms) for rapid route changes to prevent rapid state swaps
- Add routeCheckTimer tracking to properly clean up interval
- Verify page key and sequence number after async loadCurrentSession()
- Add pendingRouteChange null assignment after timeout fires
- Add MESSAGE_VERSION to all window.postMessage() calls in agent.js
- Add version checking in handlePageMessage with logging