test(peer-cli): cover streamed retry fallback

NEXT_STEPS item 5 needs streamed installs to have an explicit retry
policy. The handler already retries whole-stream attempts across the
majority-validated peer set, so add S42 to prove that behavior with the
Docker harness instead of leaving it implicit.

S42 starts two catalog-version-matching `cnctw` sources. The first source
sorts first in retry order but has `--unrar /missing-unrar`, so its stream
attempt fails before sending chunks. The second source then completes a
fresh whole-stream attempt. The scenario asserts local-only installed
state, no root archive or sentinel, no `.local.installing` staging
leftover, chunk events only from the good source, matching streamed byte
count, and SHA-256 payload equality against the good source's `unrar p`.

This pins the current policy: retry the entire stream from another
validated peer, do not preserve partial files across attempts, and do not
promise byte-offset resume.

Test Plan:
- python3 -m py_compile crates/lanspread-peer-cli/scripts/run_extended_scenarios.py
- python3 crates/lanspread-peer-cli/scripts/run_extended_scenarios.py S42
- git diff --check
- git diff --cached --check

Refs: NEXT_STEPS.md item 5

NEXT_STEPS.md

@@ -38,14 +38,13 @@ product-ready.
    step: add catalog-owned archive or extracted-file SHA-256 hashes, then verify
    those at the receiver before commit.
 
-5. **Upgrade retry/resume semantics**
+5. **Done — Upgrade retry/resume semantics**
 
-   Right now, failed stream means failed operation and rollback. Next useful
-   step:
-
-   - retry whole stream from another trusted peer
-   - later, maybe keep completed files and restart only the interrupted file
-   - avoid byte-offset resume until there’s a strong reason
+   Streamed install attempts now use the same majority-validated peer set as
+   normal downloads, and each failed attempt rolls back its staging transaction
+   before trying the next peer. S42 pins the policy: retry the whole stream from
+   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**