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/projectile_materials.json

@@ -0,0 +1,10 @@
+[
+  {
+    "name": "test-steel",
+    "density_kg_m3": 7850.0,
+    "mu_r": 200.0,
+    "b_sat_tesla": 1.8,
+    "price_per_kg": 100.0,
+    "source": "fixture"
+  }
+]