fix(hub): Search reaches each group with one offer, and a busy hub is not a dead node0.15

The other half of the 4G failure. The page negotiated every group twice: the
sweep opened a connection, read the index and closed it, then the warm-up
opened the same group again. The sweep, the warm-up and the tiles each had a
concurrency ceiling of their own, and together they went past what the hub
admits per account. Whatever the hub refused was then reported as "node
unreachable" and remembered as down, which put that group last next time.

- Every connection goes through ConnectionPool, which holds the page's one
  ceiling (six at once, sized for twenty groups on a phone) and keeps what
  the sweep opened for the tiles. A visit costs one offer per group. A
  refresh costs none for a connection that answers a four-second ping, and
  a connection that died while the phone slept is replaced, not waited on.
- A connection whose index is being read is held against eviction. With
  more groups than the pool keeps, it was otherwise the least recently used
  one.
- Negotiations still under way when the page closes close what they get,
  and a sweep cut short that way remembers nobody as down.
- transport.js sends an offer again on 429, 502 or 503, honouring
  Retry-After, with jittered waits of about twenty seconds at worst. Search
  counts each retry as progress. A 404, 403 or 504 still fails at once, so
  a dead node costs no time.

The fan-out tests assumed a ceiling of three and were re-measured: four dead
groups of twelve now hold nothing back, even on a first visit. The pool and
the retry run as shipped code, lifted as text, against a fake clock. Each
guard was checked by removing it and seeing its test fail.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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