Add desktop GUI (M10): modern dark dashboard

PySide6/Qt front-end over the stdlib sensor core (only gui/ imports Qt).
- sidebar navigation + stacked pages (Dashboard live; Logs/Health/Inventory
  placeholders for M3-M5)
- live dashboard: circular gauges (GPU temp/load, CPU temp, memory) plus
  collapsible per-subsystem cards with progress bars and metric rows
- background sampling thread -> Qt signal so the UI stays responsive
- temperature colors: icey-blue (cold) -> green -> orange -> red (hot)
- dark theme via QSS + Fusion

Supporting changes:
- cpu source: order temps as package, then cores numerically (fixes CLI too)
- render: expose format_value/metric_label, shared by CLI and GUI
- cli: `rigdoctor gui` (lazy import; prints install hint if PySide6 missing)
- pyproject: rigdoctor-gui script + [gui] extra (PySide6)
- gitignore: *.egg-info/, build/, dist/

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

docs/MODULES.md

@@ -15,7 +15,7 @@ Status: ⬜ not started · 🟦 designing · 🟨 in progress · ✅ done
 | 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 | ⬜ |
-| M10 | Desktop GUI | Desktop UI | **python3-pyside6** | 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 | ⬜ |
 | ~~M7~~ | ~~Stress / repro~~ | — | — | — | — | ❌ dropped (D7) |
@@ -35,7 +35,10 @@ Status: ⬜ not started · 🟦 designing · 🟨 in progress · ✅ done
   suggests the fix command but does not apply it (D9).
 - **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.
+  browser, report viewer, logger controls). Optional; adds the Qt dependency. *Bootstrapped
+  early (ahead of its Phase 4 slot) at the user's request:* dark-themed window with sidebar
+  nav and a live dashboard (circular gauges + collapsible per-subsystem cards, temperature-
+  colored values); Logs/Health/Inventory are placeholders until M3–M5.
 - **M11 Tray applet** — `QSystemTrayIcon` menu-bar applet. Dropdown shows live M1 readouts
   (CPU temp, GPU temp, memory used/total, status dot) and is led by a **Run Diagnostic**
   action (the guided diagnostic session), plus Open dashboard / Start-Stop recording /