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>

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "peerspeak"
-version = "0.3.0"
+version = "0.4.0"
 edition = "2024"
 # Application crate, not a crates.io library — refuse `cargo publish` and let
 # cargo-deny's [licenses.private] skip the missing-license check.