#774 Activity captured_logs intermix across concurrent activities (root-logger global handler)

BufferingActivityLogHandler attaches to the ROOT logger, so when the activity worker runs N activities concurrently (threads), each activity's captured_logs contains other activities' lines (observed 'Logged activity start: <other-id>' inside a different activity's log). Fidelity bug, pre-existing; surfaced now that activity logs are persisted (#773). Fix needs thread-local/contextvar-scoped capture so each activity only captures its own thread's records.