test: make delete and awareness deflakes deterministic
Test-only determinism work with zero production diff. Deflakes:
- solstone/apps/transcripts/tests/test_segment_routes.py::test_cancel_delete_segment_within_window_keeps_directory
- tests/test_awareness_call_parity.py::test_log_read_fetches_all_entries_past_default_cap_and_page_boundary
- tests/test_awareness_routes.py::test_awareness_imports_post_record
- tests/test_awareness_routes.py::test_awareness_imports_post_nudge
Removes two root causes: the real threading.Timer path from the transcripts delete cluster, and 175 fsync'd append_log calls plus whole-Convey create_app startup from awareness tests. Real timer and TTL semantics remain owned by tests/test_deferred_deletes.py. The faked awareness owner functions are now covered directly by persistence tests in tests/test_awareness.py.