Pace-free completion framing, stray UTF-8 flush, and late-rejection guards:
- Pre-encode the completion value at its validation point so send_done never
re-walks a live value a mutating daemon thread could change (TOCTOU); a
mutation-induced encode throw is then classified as 'exception', not a
host-side worker-exit.
- Budget-triggered stray flush retains an incomplete multibyte UTF-8 tail
(<=3 bytes) as residual instead of decoding a legal, split character to
U+FFFD in an admitted entry; the end/closeDeadline paths still full-decode.
- Check 'settled' before formatting a late binding rejection's message, so a
hostile message getter cannot stall or exhaust a run that already settled.
- Document _check_done_value's first-to-trip ruling in its docstring.
- Rewrite ProtocolChannel.send_sync around a shared write_encoded that the
done frame's pre-encoded string path uses.
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