Follow-up for the salvaged substantive-progress fix, adapted to main's
three-hook architecture:
- Keep the dispatch tick and the completed-response shim tick (both
deliberate on main; one-shot terminal events that cannot defeat an
inactivity timeout) — update the two PR assertions accordingly.
- Pin the end-to-end bug: keepalive/empty-role chunks must leave
CompressionCommitFence stale, a substantive token must refresh it.
- Pin the fast-lane telemetry contract (#96945/#96963):
time_to_first_progress_ms records on the first frame of any kind via
the split-out _notify_aux_timing_response helper.
436435bfc9