feat(game): broadcast game presence (GOSSIP_PROTO 3) + per-game background

Steps 5-6 of game detection. BREAKING wire change — bump everyone.

Wire (step 5):
- PeerState.game: Option<String> (display label only — never appid/source).
- SelfPresence.game + to_state carry it (single self-state builder).
- GOSSIP_PROTO 2->3, GOSSIP_SIG_DOMAIN v3, version comment bumped together;
  Cargo MINOR 0.3.0 -> 0.4.0 per VERSIONING.md. v2/v3 isolate into
  different topics + signature domains, so a coordinated redeploy is
  required (same as the W4 avatar bump).
- Gossip ingest sanitizes incoming game via sanitize_game_label (bidi/
  control strip, 64-char/256-byte cap); empty -> None.
- Bonus security fix (Codex find): reject inbound gossip frames over a
  128KB cap BEFORE serde_json::from_slice — a legit Announce with a full
  48KB avatar is ~49KB, so this bounds allocation abuse with headroom.

Core wiring:
- Spawns the detector at startup; consumes its watch channel in the main
  select. Detection runs continuously (for the local background); the
  broadcast is gated by game_presence_enabled (opt-in, default OFF).
  New commands: SetGamePresenceEnabled (immediate publish/clear, D8),
  SetGameOverride, SetGameProcessMap. New event: GameChanged.
- game_presence_label sanitizes the outgoing label too.

Background switch (step 6):
- GUI handles GameChanged: stores current_game, swaps background to the
  per-game override (config.game_backgrounds[id]) or falls back to the
  W16 default; reuses the existing cached-handle path (no redraw flicker).

397 lib tests (all green), clippy --all-targets clean, full binary builds.
Remaining: step 7 UI (opt-in toggle, roster 'Playing' text, manual
override control, Settings game-backgrounds + process-map editors).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -381,6 +381,11 @@ pub struct AppState {
     /// doesn't read the file from disk on every redraw. Loaded on startup and
     /// refreshed when the background is changed/removed. `None` = no custom bg.
     background_image: Option<bytes::Bytes>,
+    /// The locally-detected running game (game detection), as reported by core.
+    /// Drives the per-game background (W18) and a local "Playing …" indicator.
+    /// `None` = nothing detected. Independent of whether we broadcast it to peers
+    /// (that's `config.game_presence_enabled`).
+    current_game: Option<crate::game::DetectedGame>,
     peers: HashMap<EndpointId, PeerState>,
     audio_levels: HashMap<EndpointId, f32>,
     /// Peers we've locally muted (their audio isn't mixed into our output).
@@ -565,6 +570,7 @@ impl Default for AppState {
             selected_output,
             config,
             background_image,
+            current_game: None,
             peers: HashMap::new(),
             audio_levels: HashMap::new(),
             locally_muted: HashSet::new(),
@@ -624,6 +630,23 @@ fn load_background_bytes(config: &AppConfig) -> Option<bytes::Bytes> {
     std::fs::read(path).ok().map(bytes::Bytes::from)
 }
 
+/// The effective background for the current state: the per-game override (W18) when
+/// the running `game` has a mapping in `config.game_backgrounds`, otherwise the
+/// single custom background (W16). A mapped-but-missing/unreadable per-game file
+/// falls back to the default WITHOUT forgetting the mapping (the file may return).
+fn effective_background_bytes(
+    config: &AppConfig,
+    game: Option<&crate::game::DetectedGame>,
+) -> Option<bytes::Bytes> {
+    if let Some(g) = game
+        && let Some(path) = config.game_backgrounds.get(&g.id)
+        && let Ok(bytes) = std::fs::read(path)
+    {
+        return Some(bytes::Bytes::from(bytes));
+    }
+    load_background_bytes(config)
+}
+
 pub fn run_gui() -> iced::Result {
     // Restore the last window size (saved on close). Position is restored too,
     // but only on X11 — Wayland's xdg-shell gives clients no way to set their own
@@ -1144,6 +1167,15 @@ fn update(state: &mut AppState, message: AppMessage) -> Task<AppMessage> {
                         format!("Presence mode stayed {mode}")
                     };
                 }
+                UiEvent::GameChanged(detected) => {
+                    // The locally-detected game changed: switch the per-game
+                    // background (W18) if one is mapped, else fall back to the
+                    // default. Presence broadcasting is handled in core, gated by
+                    // the opt-in toggle; this is purely local presentation.
+                    state.current_game = detected;
+                    state.background_image =
+                        effective_background_bytes(&state.config, state.current_game.as_ref());
+                }
                 UiEvent::ShutdownComplete => {
                     if state.closing {
                         return iced::exit();