sprint-0: foundations -- data model, seed library, tests, demo

Initial commit. Delivers what the brief calls 'Sprint 0 - Foundations'
(see docs/AR_Autopilot_brief.md section 12):

- Complete repository structure (arautopilot package + firmware, display,
  installer, tools placeholders + docs).
- Core data model (Pydantic v2): modes, alarms, actuator config, PID
  config + gain scheduling, vessel config, knob state machine, project
  config with YAML/JSON serialisation.
- Seed library: 2 actuator profiles (hydraulic & electric DC reversible)
  and 2 default tunings (yacht motor planeo 30 m and 40 m). Conservative
  literature values, NOT the integrator's production tuning IP.
- Firmware skeleton: only src/hal/pinout.h with the 21 I/O contract for
  the AR-NMEA-IO v1.0 board. No drivers, no main loop.
- Studio stubs (real PySide6 app starts in Sprint 4).
- pytest suite (80 tests, all green): modes, alarms, actuator, PID
  (incl. gain interpolation and the +/-50% adaptive bound from brief
  section 6), vessel, knob state, project config, library loader,
  end-to-end roundtrip.
- examples/sprint0_demo.py - the acceptance demo from the brief.

Acceptance criteria met:
- pytest green (80/80)
- demo creates, saves (YAML + JSON), reloads, and verifies a full
  ProjectConfig using the seed library
- repository ready for tag `sprint-0-approved`

See CHANGELOG.md for the detailed scope.

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

arautopilot/studio/app.py

@@ -0,0 +1,25 @@
+"""Studio application entry point — Sprint 4 stub.
+
+This module is intentionally a stub. The real PySide6 ``QApplication`` and
+``MainWindow`` arrive in Sprint 4. Trying to launch the Studio now will
+print a friendly notice and exit cleanly.
+"""
+
+from __future__ import annotations
+
+import sys
+
+
+def run() -> int:
+    """Stub entry point. Will be replaced by a real ``QApplication`` in Sprint 4."""
+    print(
+        "AR-Autopilot Studio — Sprint 0 stub.\n"
+        "The Studio GUI is implemented starting in Sprint 4.\n"
+        "For now, use the core API (`arautopilot.core`) and the demo:\n"
+        "    python examples/sprint0_demo.py"
+    )
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(run())