refactor(peer): split local library and operation UI events

Replace the `a9f9845` local-update dedup cache with explicit peer event
semantics. Local scans now emit `LocalLibraryChanged` when the library changes,
while operation mutations emit `ActiveOperationsChanged` from the mutation
path. Tauri keeps joining those facts into the existing `games-list-updated`
payload, so the frontend contract stays stable.

This removes the cache/invalidation coupling between scan emission and
operation state. The remaining forced local snapshot is explicit: accepted game
directory changes can refresh the UI for an equivalent new path without sending
a peer library delta.

Operation guard cleanup and liveness cancellation now publish the same active
operation snapshot as normal command-handler transitions. The peer CLI JSONL
events follow the same split with `local-library-changed` and
`active-operations-changed`.

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

Refs: CLEAN_CODE_PLAN_1.md

crates/lanspread-peer/ARCHITECTURE.md

@@ -77,6 +77,11 @@ When a peer is discovered:
   - an active operation lock drops events for that game;
   - a rescan already running for the ID sets a rescan-pending flag;
   - the running rescan loops once more when that flag was set.
+- Local library scans emit `LocalLibraryChanged` only for real library changes,
+  except that accepted game-directory changes can force a UI snapshot for the
+  new path without sending a peer delta.
+- Active operation mutations emit `ActiveOperationsChanged` from the mutation
+  path instead of riding on local library scans.
 - Send `LibraryDelta` to known peers; send `LibrarySummary` on new connections.
 
 ## Local game scanning: fast and low cost