feat: Steam game & library detection (M6) — 0.8.0

The first slice of M6 (gaming-environment checks): detect a user's Steam
libraries and the games installed in each — also the D12 "pick a game"
foundation.

- core/steam.py: multi-install/library discovery (libraryfolders.vdf, symlink
  dedupe, native/Flatpak/Snap), appmanifest_*.acf scan with runtime/Proton/
  redist filtering, scan cache + new-game diff. Stdlib only. VDF keys read
  case-insensitively (e.g. lastupdated vs SizeOnDisk).
- Libraries are opt-in (config steam_libraries); the flat TOML writer now
  emits list/array values.
- GUI Games page: library checkboxes with per-library counts, game list,
  background rescan on every launch, NEW badge + sidebar count for games
  installed since the last scan (acknowledged when viewed).
- CLI: rigdoctor games / games libraries [--enable|--disable|--all|--json]
  (headless-complete, D17).
- Tests for VDF parse, scan, tool filter, cache diff, config list round-trip.
- Docs (MODULES/ROADMAP) updated; version 0.7.3 -> 0.8.0.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/MODULES.md

@@ -14,7 +14,7 @@ Status: ⬜ not started · 🟦 designing · 🟨 in progress · ✅ done
 | M2 | Live monitor (TUI) | Monitoring | none (stdlib curses) | all | P1 | ⬜ |
 | M8 | Alerting | Monitoring | libnotify (opt) | all | P2 | 🟨 |
 | M5 | System inventory | Diagnostics | none (opt: lm-sensors, dmidecode) | all | P1 | 🟨 |
-| M6 | Gaming env checks | Diagnostics | none | all | P2 | ⬜ |
+| M6 | Gaming env checks | Diagnostics | none | all | P2 | 🟨 |
 | M10 | Desktop GUI | Desktop UI | **python3-pyside6** | all | P2 | 🟨 |
 | M11 | Tray / menu-bar applet | Desktop UI | **python3-pyside6** (+ AppIndicator on GNOME) | all | P2 | ⬜ |
 | M9 | Installer | (meta) | none | all | P1 | 🟨 |
@@ -41,7 +41,15 @@ Status: ⬜ not started · 🟦 designing · 🟨 in progress · ✅ done
   (text/JSON) + GUI Health tab. GPU-firmware verification deferred.
 - **M2 Live monitor** — depends on M1; the terminal "HWMonitor for Linux" face. Stdlib-only.
 - **M5 / M6 Diagnostics** — inventory export + gaming-env checks; M6 flags risky settings and
-  suggests the fix command but does not apply it (D9).
+  suggests the fix command but does not apply it (D9). *M6 implemented (Steam detection first —
+  the D12 "pick a game" foundation):* discovers Steam installs + all library folders
+  (`libraryfolders.vdf`, multi-drive) and the games in each (`appmanifest_*.acf`), filtering
+  runtimes/Proton/redistributables — stdlib only. **Libraries are opt-in** (`steam_libraries`
+  config); the GUI **Games** page lists them with per-library counts and rescans in the
+  background on every launch, badging games installed since the last scan (cached in
+  `state/games.json`). CLI: `rigdoctor games` / `games libraries [--enable|--disable|--all]`.
+  *Pending:* the env-check probes (CPU governor, GPU persistence, GameMode/MangoHud, swappiness,
+  hugepages, mitigations, PCIe ASPM) and non-Steam launchers (Lutris/Heroic).
 - **M8 Alerting** — threshold/event notifications; integrates with the tray applet (M11).
 - **M10 Desktop GUI** — PySide6 graphical front-end over the core engine (dashboard, log
   browser, report viewer, logger controls). Optional; adds the Qt dependency. *Bootstrapped

docs/ROADMAP.md

@@ -27,7 +27,11 @@ Ubuntu + NVIDIA first; `.deb` distribution (see `DECISIONS.md`).
 
 ## Phase 3 — Diagnostics breadth
 - [ ] M5 system inventory + exportable report
-- [ ] M6 gaming environment checks (suggest-only)
+- [~] M6 gaming environment checks (suggest-only) — *Steam game/library detection done*
+      (multi-library `libraryfolders.vdf` discovery + `appmanifest` scan, opt-in libraries,
+      launch-time background rescan with new-game badge; CLI `rigdoctor games`, GUI Games page).
+      This is also the D12 "pick a game" foundation. *Pending:* the env-check probes (governor,
+      GPU persistence, GameMode/MangoHud, swappiness, hugepages, mitigations, PCIe ASPM).
 - [ ] SMART integration (smartmontools if present)
 
 ## Phase 4 — Desktop UI & installer