Fixed potential use-after-free in Update() when gcRound deletes
a session between GetOrCreate() and acquiring the session lock.
Changes:
- Add 'deleted' flag to SessionState
- Mark sessions as deleted before removing from map in gcRound
- Check deleted flag in Update and recreate session if needed
This ensures updates to deleted sessions create a new session
instead of modifying a freed/dangling reference.
Race detector verified: go test ./internal/correlation/... -race
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
678aa48a12