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>

README.md

@@ -73,7 +73,21 @@ PYTHONPATH=src python3 -m rigdoctor sources       # list detected sensor sources
 PYTHONPATH=src python3 -m unittest discover -s tests
 ```
 
-Or `pip install -e .` to get a `rigdoctor` command on your PATH.
+### Desktop GUI (M10)
+
+The GUI uses PySide6 (Qt) — the only part of RigDoctor that needs a non-stdlib dep:
+
+```bash
+pip install -e '.[gui]'   # core + PySide6, gives `rigdoctor` and `rigdoctor-gui`
+rigdoctor gui             # or: rigdoctor-gui
+```
+
+It opens a dark-themed window with sidebar navigation and a **live dashboard** over the
+same sensor core — circular gauges for the headline metrics plus collapsible per-subsystem
+cards (GPU/CPU/memory/storage) with temperature-colored values (icey-blue → green → red).
+The Logs / Health / Inventory sections are placeholders until M3–M5 land.
+
+Without the GUI extra, `pip install -e .` gives just the stdlib-only CLI.
 
 ## Start here