While headroom is unhealthy, skip it only for sessions with no warm cache, and rejoin on sustained health #399

A session already using headroom keeps replaying through it on a reset: a bypassed request sends the uncompressed prefix and rewrites the cache. A new session, or one at a cache boundary, has no cache to lose, so while the daemon is unhealthy it can go direct and rejoin headroom once the daemon has been healthy for a sustained stretch, so a skipped session does not stay off headroom for its whole life.

The health signal and the length of the stretch come from the door's own hop outcomes (resets, replays that fail, refused hops), not a fixed timer. Today a hop to an unreachable daemon answers 502 rather than bypassing; this changes that only for sessions at a boundary.