Add component database layer and validated physics core

- components/schema.py + database.py: typed loader for real retail parts
  (wire, capacitors, switches, sensors, projectile materials)
- physics/inductance.py: multilayer solenoid inductance (Wheeler) with
  ferromagnetic-slug coupling via smooth overlap model and analytic dL/dx
- physics/force.py: F = 0.5*I^2*dL/dx with a saturation clamp
- physics/circuit.py: coupled [Q,I,x,v] discharge ODE
- Validated against textbook RLC analytical solutions (under/over/critically
  damped) and energy conservation, not just spot-checked by eye
- PLAN.md updated with animation (per-run field visualization) and
  GPU-accelerated batch sweep as explicit later stages

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/fixtures/data/sensors.json

@@ -0,0 +1,18 @@
+[
+  {
+    "part_number": "test-optical",
+    "kind": "optical",
+    "propagation_delay_ns": 500.0,
+    "price": 20.0,
+    "source": "fixture"
+  },
+  {
+    "part_number": "test-inductive-pickup",
+    "kind": "inductive",
+    "propagation_delay_ns": 0.0,
+    "price": 5.0,
+    "source": "fixture",
+    "sensitivity_v_per_mps": 0.02,
+    "threshold_v": 0.1
+  }
+]