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/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