A stalled compression summary never raises, so the auxiliary client's
exception-path fallback is unreachable from it. When the progress-aware
timeout aborts a stalled worker, re-run the summary once pinned to the
first auxiliary.compression.fallback_chain entry before degrading to
continue-without-compression.
The pin is a single-use ContextVar consumed by the context compressor's
summary call, so it cannot leak into the detached stalled worker or the
compressor's own main-model retry. A fresh fence is minted through the
host factory so a /stop during the retry still admits against the live
commit boundary.
2c6938dc3a