P2: true multi-axis overlay + gauge view

gui/widgets.py:
- MultiAxisPlot -- overlay with one Y axis PER UNIT (psi/V/rpm/C/%), linked
  ViewBoxes on X, so mixed-scale signals are readable at true values
  (base left axis + up to 4 right axes).
- SinglePlot -- one shared axis for the Normalize (% of range) mode.
- ArcGauge -- 270deg arc gauge with peak tick + numeric readout, own dark bg.
- GaugeGrid -- scrollable grid of gauges.

gui/main.py:
- Graph page is now a multi-axis/single-axis sub-stack; Normalize toggles
  between true multi-axis (raw) and single-axis (%). curves map key->color;
  plot ops route to the active graph widget.
- Gauge View menu enabled (3rd center page); gauges update on tick with peak.
- Theme applies to both plot widgets; profile switch clears graphs/gauges.

Fix: ArcGauge QPen built via setCapStyle (the QPen(...cap=...) kwarg segfaults
PySide6). Validated headless: driving preset -> 6 unit groups across 5 axes,
gauge view renders, normalize round-trips, profile-switch clears cleanly.
obdcore + diagnostics tests still pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016yT89n4zR4qbrySoSiEyZs

README.md

@@ -73,21 +73,31 @@ python obd_reader.py COM5 --crank --dash-log crank.csv   # + record a CSV
 under the line), that's the high-pressure oil bleed-off — STC fitting / oil-rail
 O-rings. On exit it prints the peak and a verdict. `q` quits, `r` resets.
 
-## Graphical app (preview — P1)
+## Graphical app (preview)
 
-A cross-platform desktop GUI (PySide6 + pyqtgraph) is in progress. P1 = PID
-browser + live overlay plot; see [ARCHITECTURE.md](ARCHITECTURE.md) for the
-roadmap (cranking/driving/diagnostics perspectives, record/playback, etc.).
+A cross-platform desktop GUI (PySide6 + pyqtgraph). Vehicle-agnostic — all PIDs,
+scaling, DTCs, and presets come from the JSON profiles in `profiles/`.
 
 ```
 pip install -r requirements-gui.txt
 python run_gui.py        # tick "Mock" + Connect to explore with no adapter
 ```
 
-![P1 GUI](docs/gui-p1-preview.png)
+Features so far:
+- **PID browser** (left) grouped by system, live values, confidence badges
+- **Graph view** with **true multi-axis** overlay — one Y scale per unit (psi/V/rpm/…),
+  or a Normalize (% of range) mode
+- **Gauge view** — arc gauges with peak-hold, one per signal
+- **Table view** — value + min/max + confidence
+- **Diagnostics** — read/clear DTCs (guarded), no-start codes flagged
+- **Profile menu** — switch/import/edit vehicles; **File menu** — record/replay/export captures
 
-The whole app runs against simulated data (`MockLink`) so it can be developed
-on any machine and only needs the truck for real captures.
+![Multi-axis graph](docs/gui-p2-multiaxis.png)
+![Gauge view](docs/gui-p2-gauges.png)
+
+The whole app runs against simulated data (`MockLink`), so it can be developed
+on any machine and only needs the vehicle for real captures. See
+[ARCHITECTURE.md](ARCHITECTURE.md) for the roadmap.
 
 ---