Corrections to the settlement-path review fixes:
- send_done was invoking channel.send_sync / channel.write_encoded via a late
method look-up, which a program running as __main__ could rebind through
__main__.ProtocolChannel.send_sync before the failure path ran — a rebound
send that raises then skipped the done frame and downgraded a settled
exception to worker-exit. Bind both channel methods into locals before the
program runs, mirroring the pre-existing binding of flush_out/flush_err/
safe_model_traceback.
- Restructure flushStray so the mid-sequence budget-flush retention arm is a
self-contained v8-ignored branch and the covered default path decodes the
full residual (not schedulable-through-the-seam boundary).