fix(peer): harden streamed install lifecycle

Claude Fable 5's branch review found that receiver cancellation or a QUIC
send failure could leave the sender-side archive producer blocked on the
bounded frame channel. That kept the outbound transfer guard alive and could
block later installs or updates of the same game.

Route archive frames through a cancellable StreamInstallFrameSink instead of
exposing the raw channel sender to providers. The QUIC forwarder now cancels
and closes the receive side before awaiting the producer, so a blocked send
wakes and the transfer guard can drop normally.

Make PeerCommand::StreamInstallGame own its peer metadata preflight inside the
peer core. The Tauri layer now sends the command directly, and the peer runtime
fetches file details from catalog-version peers before running the existing
majority validation and retry logic. This removes the UI-only pending streamed
install set and gives PeerEvent::GotGameFiles one meaning again: continue a
normal archive download.

Tighten the receiver transaction edge cases too. Rollback removes a newly
created empty game root, but preserves pre-existing roots. Once streamed
staging has been promoted to local/, intent or launch-settings cleanup failures
are logged for startup recovery instead of reporting a failed install for bytes
that are already committed.

Accept missing RAR CRC32 metadata for zero-byte files as CRC32 00000000 while
still requiring CRC32 metadata for non-empty files. Update the peer README,
scenario docs, and next-steps handoff so the documented ownership and remaining
trust limitation match the implementation.

Test Plan:
- just fmt
- just test
- just frontend-test
- just clippy
- git diff --check
- python3 -m py_compile \
  crates/lanspread-peer-cli/scripts/run_extended_scenarios.py
- python3 crates/lanspread-peer-cli/scripts/run_extended_scenarios.py \
  S39 S40 S41 S42 S43 S44 S45 S46 S47 --build-image

Refs: streamed-install review handoff from Claude Fable 5

NEXT_STEPS.md

@@ -61,5 +61,6 @@ product-ready.
    modal status shows `Installed, not shareable`. Downloaded-and-installed games
    keep the normal `Installed` label.
 
-My recommended next slice: make the provider abstraction final-ish, then
-implement a real one-pass provider. Everything else builds cleanly on that.
+The remaining production-readiness step is additive: move from sender-owned RAR
+metadata to catalog-owned archive or extracted-file hashes, then verify those
+at the receiver before committing the streamed install.