feat(game): Steam + process-scan OS adapters

Step 2-3 of game detection (game-presence-plan.md). The OS edges feeding
the pure seams from the previous commit.

- src/game/steam.rs: SteamProbe — reads the live RunningAppID and resolves
  it to a name via appmanifest_<id>.acf (no binary appinfo.vdf). Pure parse
  fns (parse_running_app_id / parse_library_paths / parse_app_name) over
  file contents are unit-tested incl. current+legacy libraryfolders shapes,
  escaped Windows paths, empty/missing names, and garbage. Roots discovered
  across native/Flatpak/Snap (Linux) and the registry (Windows); libraries
  and resolved names cached + mtime-invalidated so the 3s poll doesn't
  rescan. File reads byte-capped.
- src/game/scan.rs: native running-process enumeration — /proc (exe symlink,
  comm fallback) on Linux, Toolhelp on Windows — feeding the pure
  match_processes. No sysinfo dep (D7).
- Cargo.toml: windows-sys as a direct Windows-only dep for the registry +
  Toolhelp FFI. No NEW crate — it was already in the lockfile transitively
  via cpal/rfd, so the audit surface is unchanged.

391 lib tests (+5). Linux: build + clippy --all-targets clean. Windows FFI
signatures verified against windows-sys 0.61 source (one *const vs *mut
lpReserved fixed) but NOT yet cross-compiled — defer to the post-UI Windows
build cycle.

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

Cargo.toml

@@ -65,3 +65,12 @@ rfd = { version = "0.17", default-features = false }
 # Windows audio backend: cpal drives WASAPI for capture/playback behind the
 # AudioBackend trait (src/audio/cpal_impl.rs). The Linux counterpart is pipewire.
 cpal = "0.15"
+# Win32 FFI for game detection (no new crate: windows-sys is already pulled in
+# transitively by cpal/rfd). Registry reads the Steam RunningAppID + install path;
+# Toolhelp enumerates running processes for the non-Steam process-scan fallback.
+windows-sys = { version = "0.61", features = [
+    "Win32_Foundation",
+    "Win32_System_Registry",
+    "Win32_System_Diagnostics_ToolHelp",
+    "Win32_System_Threading",
+] }