feat: gaming environment checks engine (M6) + notification icon — 0.9.0

The evaluate-and-suggest half of M6: a read-only findings report (D9) over
system settings that affect gaming stability/performance, each with the exact
fix command.

- core/gameenv.py: PCIe ASPM, NVIDIA persistence mode, CPU governor (the three
  seed-case contributors to GPU bus-drop / Xid 79), GameMode, MangoHud,
  vm.swappiness, shader disk cache, THP, CPU mitigations, Proton versions.
  Pure evaluate_* helpers split from IO for testing; reuses the M4 Finding model.
- steam.proton_versions(): surfaces installed Proton builds for the report.
- CLI: rigdoctor gameenv (text / --json); render_health() gained a title arg.
- GUI: new Environment page; extracted a shared finding_card widget and switched
  the Health page to it.
- Tests for the pure evaluators + aggregate.

Also fix: desktop notifications now use the RigDoctor icon (installed theme copy
-> bundled asset -> stock fallback) instead of a generic stock icon, matching
the app/dock icon.

Docs (MODULES/ROADMAP) updated; version 0.8.0 -> 0.9.0.

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

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "rigdoctor"
-version = "0.8.0"
+version = "0.9.0"
 description = "Modular hardware monitoring & crash diagnostics for Linux gamers."
 readme = "README.md"
 requires-python = ">=3.11"