P1: PySide6 + pyqtgraph GUI shell (PID browser + live overlay plot)

First graphical frontend on obdcore. Cross-platform (Win/mac/Linux).

- gui/controller.py: owns link/registry/store/scheduler; subscribe == poll ==
  plot; per-PID rates (ICP/FICM/RPM fast); optional CSV recording.
- gui/main.py: connection bar (port dropdown via find_ports, baud, Mock,
  connect), left PID browser grouped by system with live values + confidence
  badges + checkboxes, central pyqtgraph overlay plot with legend, Normalize
  (% of range) toggle for mixed-scale PIDs, Crank/Driving/Vitals presets,
  10Hz refresh reading the store off the acquisition thread.
- run_gui.py launcher; requirements-gui.txt.
- store.py: lock Channel push/series (GUI reads while scheduler writes).
- docs/gui-p1-preview.png: validated render (mock crank, ICP ramp to 540).

Validated headless (offscreen Qt): connect(mock) -> crank preset -> ICP
streams past 500 -> normalize -> uncheck removes curve -> clean disconnect.
obdcore tests still pass after the locking change.

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

requirements-gui.txt

@@ -0,0 +1,7 @@
+# GUI dependencies (cross-platform: Windows / macOS / Linux, incl. Apple Silicon)
+#   pip install -r requirements-gui.txt
+#   python run_gui.py
+PySide6>=6.6
+pyqtgraph>=0.13
+numpy>=1.24
+pyserial>=3.5