test(peer-cli): expand streamed install edge coverage

NEXT_STEPS item 6 called for the remaining streamed-install edge cases to
be covered in the peer-cli matrix. Add S43-S47 for already-installed
rejection, corrupt archive rollback, sender disconnect, receiver cancel,
and sorted multi-archive streaming.

The receiver-cancel scenario needs the harness to drive the same runtime
path as the GUI, so `lanspread-peer-cli` now accepts a narrow
`cancel-download` command that forwards to `PeerCommand::CancelDownload`.
A parser test covers the new JSONL command shape.

Add `fixture-multi/cnctw`, a tiny two-archive RAR fixture. S47 uses it to
prove streamed installs process root `.eti` archives in sorted order and
commit only extracted `local/` payloads, not the root archives or
`version.ini` sentinel.

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

Refs: NEXT_STEPS.md item 6

NEXT_STEPS.md

@@ -46,15 +46,13 @@ product-ready.
    another validated peer, keep no partial files across attempts, and do not add
    byte-offset resume until there is a strong reason.
 
-6. **Expand scenario coverage**
+6. **Done — Expand scenario coverage**
 
-   I’d add cases for:
-
-   - sender disconnect mid-stream
-   - receiver cancel mid-stream
-   - corrupted/truncated stream fails and leaves no `local/`
-   - already-installed game rejects streamed install
-   - multi-archive `.eti` roots stream in sorted order
+   S43-S47 cover the remaining streamed-install edges: already-installed
+   rejection, corrupt archive rollback, sender disconnect mid-stream, receiver
+   cancel mid-stream, and multi-archive `.eti` roots streamed in sorted order.
+   The peer-cli harness now exposes `cancel-download` so cancellation scenarios
+   exercise the same runtime path as the GUI.
 
 7. **Clean product semantics**