feat(peer): add transactional local game operations

Implement the peer-owned state model from PLAN.md. A root-level version.ini
is now the download completion sentinel, local/ as a directory is the install
predicate, and exact root-level version.ini detection prevents nested files
from becoming sentinels by accident.

Add the peer operation table that gates downloads, installs, updates, and
uninstalls by game ID. Serving paths now reject non-catalog games, active
operations, missing sentinels, and any request that points under local/.
Remote aggregation treats LocalOnly peers as non-downloadable so they do not
contribute peer counts, candidate source selection, or latest-version checks.

Move install-side filesystem mutation into lanspread-peer::install. The new
module writes atomic .lanspread.json intents, uses .local.installing and
.local.backup with .lanspread_owned markers, and performs startup recovery
from recorded intent plus filesystem state. Downloads now buffer version.ini
chunks in memory and commit the sentinel last through .version.ini.tmp.

Replace the fixed 15-second monitor with notify-backed non-recursive watches,
per-ID rescan gating, and a 300-second fallback scan. The optimized rescan
path updates one cached library-index entry and active operation IDs preserve
their previous summary during scans.

Test Plan:
- just fmt
- just clippy
- just test
- just build

Refs: PLAN.md

crates/lanspread-peer/src/remote_peer.rs

@@ -26,6 +26,12 @@ pub async fn ensure_peer_id_for_addr(
 }
 
 pub fn summary_from_game(game: &Game) -> GameSummary {
+    let availability = if game.downloaded {
+        Availability::Ready
+    } else {
+        Availability::LocalOnly
+    };
+
     GameSummary {
         id: game.id.clone(),
         name: game.name.clone(),
@@ -34,7 +40,7 @@ pub fn summary_from_game(game: &Game) -> GameSummary {
         installed: game.installed,
         eti_version: game.eti_game_version.clone(),
         manifest_hash: 0,
-        availability: Availability::Ready,
+        availability,
     }
 }