Treat cancellation as part of the Stream Install transport contract. Frame
production and QUIC egress now run as structured futures, cancellation wins at
queued sends, blocked writes, and close, and exceptional exits reset the send
stream before producer cleanup completes.
Make the unrar listing subprocess cancellation-aware and explicitly kill and
reap it on cancellation or pipe-capture failure. This ensures outbound transfer
tracking is cleared only after provider work is quiescent, which is required by
game-root mutation and directory-switch draining.
Test Plan:
- `just clippy` -- passed
- `just test` -- passed (242 lanspread-peer tests)
- `just peer-cli-build` -- passed
- `git diff --cached --check` -- passed
ddidderr
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