fix(peer): settle current-protocol local state cleanup

The follow-up backlog had drifted into three settled peer/runtime issues: the
legacy game-list fallback contradicted the one-wire-version policy, the Tauri
shell still re-derived local install state from disk after peer snapshots, and
`Availability::Downloading` existed even though active operations are already
reported through a separate operation table.

Remove the legacy `AnnounceGames` request and fallback service. Discovery now
ignores peers that do not advertise the current protocol and a peer id, and
library changes are sent through the current delta path only. This keeps the
runtime aligned with the documented current-build-only interoperability model.

Make peer `LocalGamesUpdated` snapshots authoritative for local fields in the
Tauri database. The GUI-side catalog still owns static metadata such as names,
sizes, and descriptions, but downloaded, installed, local version, and
availability now come from the peer runtime instead of a second whole-library
filesystem scan. Snapshot reconciliation also pins the missing-begin and
missing-finish lifecycle cases in tests.

Collapse availability back to the settled `Ready` and `LocalOnly` states.
Aggregation now counts only `Ready` peers as download sources, and the frontend
no longer carries a dead `Downloading` enum value.

The core peer also exposes the small non-GUI hooks needed by scripted callers:
startup options for state and mDNS, a local-ready event, direct connection, peer
snapshots, and an explicit post-download install policy. Those hooks reuse the
same current protocol path and do not add compatibility shims.

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

Refs: BACKLOG.md, FINDINGS.md, IMPL_DECISIONS.md

IMPL_DECISIONS.md

@@ -25,9 +25,12 @@
   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.
+- Removed the unreachable `Availability::Downloading` protocol value. Active
+  operations are reported separately, and local summaries emit only settled
+  availability.
 - Threaded availability through the UI-facing `Game` payload so `LocalOnly`
   rendering follows backend state instead of reverse-engineering it from
   `installed && !downloaded`.
+- Removed Tauri's parallel whole-library filesystem scan. The UI database keeps
+  bundled catalog metadata, while peer `LocalGamesUpdated` snapshots now own
+  `downloaded`, `installed`, `local_version`, and `availability`.