Fixes the bug where watcher stayed stuck on old sessions after /new or /reset.
Changes:
- Added file_score() function combining mtime + size for better detection
- Added INACTIVITY_THRESHOLD (30s) - if no new data, check for active session
- Tracks last_data_time and file size to detect stale sessions
- Switches to newer session when current is inactive
The previous v1.1 fix (mtime polling) was incomplete because new sessions
can have older mtime than recently-written old sessions.
Tested: Watcher now properly follows session rotation on /new and /reset
5c2014cb11