fix(peer): repair update lifecycle regressions

FINDINGS.md identified three merge blockers in the post-plan install/update
flow.

Updates now use FetchLatestFromPeers so the Tauri update command bypasses
local manifest serving and asks peers that advertise the latest version for
fresh file metadata. PeerGameDB now aggregates and validates file descriptions
from latest-version peers, keeping stale cached metadata for older versions
from poisoning chunk planning when filenames stay the same but sizes change.

Download-to-install handoff now performs explicit async state transitions.
The download task mutates Downloading to Installing or Updating under the
active-operation write lock, clears the cancellation token, and then runs the
install transaction. OperationGuard remains armed only as crash or abort
cleanup and is disarmed after normal explicit cleanup, so final refreshes no
longer race a deferred Drop cleanup.

Local library index writers now serialize the load/mutate/save window with one
async mutex. The index fingerprint also includes the root version.ini contents
so a same-length version rewrite in the same mtime second still updates the
reported local version.

The tradeoff is that local index mutations are serialized in-process instead
of moved into a dedicated actor. That keeps the fix small and scoped to the
merge blockers while preserving the existing scanner API.

Test Plan:
- just fmt
- just test
- just clippy
- just build
- git diff --check

Refs:
- FINDINGS.md

IMPL_DECISIONS.md

@@ -22,8 +22,9 @@
   Tauri by sending `PeerCommand::InstallGame` directly. A not-downloaded game
   still uses `GetGame`, and the peer auto-installs after the sentinel commit.
 - Removed the dead internal `PeerCommand::UpdateGame` path. The UI update button
-  intentionally sends `GetGame`, and the peer infers install versus update from
-  the presence of `local/` after archives are available.
+  now sends `FetchLatestFromPeers`, which skips local manifest serving and asks
+  latest-version peers for fresh file metadata before the normal download and
+  update transaction runs.
 - Kept `Availability::Downloading` in the wire protocol for compatibility, but
   local summaries do not emit it today because active operations are gated out
   of scans and serving decisions.