feat: system inventory (M5)

CPU, GPU (model/driver/VBIOS/VRAM/PCIe), motherboard/BIOS, RAM (total +
modules), storage (real disks only), kernel, and display server.

- core/inventory.py: collect() + Markdown/JSON/text renderers + dict round-trip;
  stdlib + nvidia-smi/lspci/lsblk/dmidecode, all degrading gracefully
- cli: `rigdoctor inventory` (--json / --markdown / -o)
- gui: Inventory tab (fills the last empty tab) with Copy-as-Markdown, Save, and
  "Run with admin" (pkexec) for dmidecode board/BIOS/RAM details
- tests for collect/render/round-trip; remove unused placeholder-page helper

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

docs/MODULES.md

@@ -13,7 +13,7 @@ Status: ⬜ not started · 🟦 designing · 🟨 in progress · ✅ done
 | M4 | Health report (log scan) | Essential | none (opt: smartmontools) | all (NVIDIA first) | P0 | 🟨 |
 | 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 | ⬜ |
+| 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 | 🟨 |
 | M11 | Tray / menu-bar applet | Desktop UI | **python3-pyside6** (+ AppIndicator on GNOME) | all | P2 | ⬜ |