Post-1.3: TUI latency shows periodic blocking/jitter pattern

Manual 1.3 testing observed the TUI latency indicator cycling through a pattern like `1ms -> 10ms -> 14ms -> 1ms` rather than staying consistently near the fast path.

This looks like a periodic blocking cycle or coarse refresh/backoff somewhere in the TUI/service polling path, but root cause is not confirmed.

Expected follow-up:
- Measure TUI route calls, terminal buffer polling, and session-list refresh separately.
- Verify whether any route still performs avoidable DB/disk work on the hot path.
- Add a latency/jitter regression gate for key TUI-facing routes.
- Keep the expected steady-state path close to 1-2ms on local service calls.

Post-1.3 unless manual release testing shows it blocks basic terminal use.

Start by measuring TUI route calls, terminal buffer polling, and session-list refresh separately to identify the periodic blocking or jitter source. Check whether route calls perform avoidable database or disk work, then add a latency/jitter regression gate for key TUI-facing routes and verify local service calls remain near 1–2ms.

Written by the indexing model from the issue text.