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/test_database_loading.py

@@ -0,0 +1,45 @@
+from pathlib import Path
+
+from gausse.components.database import ComponentDatabase
+
+FIXTURES = Path(__file__).parent / "fixtures" / "data"
+
+
+def test_loads_all_component_types():
+    db = ComponentDatabase.load(FIXTURES)
+    assert len(db.wires) == 2
+    assert len(db.capacitors) == 1
+    assert len(db.switches) == 1
+    assert len(db.sensors) == 2
+    assert len(db.projectile_materials) == 1
+
+
+def test_wires_by_material_filters_correctly():
+    db = ComponentDatabase.load(FIXTURES)
+    copper = db.wires_by_material("copper")
+    assert len(copper) == 1
+    assert copper[0].part_id == "test-cu-0.5"
+
+
+def test_capacitors_rated_at_least_filters_by_voltage():
+    db = ComponentDatabase.load(FIXTURES)
+    assert len(db.capacitors_rated_at_least(400.0)) == 1
+    assert len(db.capacitors_rated_at_least(500.0)) == 0
+
+
+def test_switches_rated_for_current_and_voltage():
+    db = ComponentDatabase.load(FIXTURES)
+    assert len(db.switches_rated_for(20.0, 500.0)) == 1
+    assert len(db.switches_rated_for(30.0, 500.0)) == 0
+
+
+def test_sensors_by_kind():
+    db = ComponentDatabase.load(FIXTURES)
+    assert len(db.sensors_by_kind("optical")) == 1
+    assert len(db.sensors_by_kind("inductive")) == 1
+
+
+def test_missing_data_dir_returns_empty_database(tmp_path):
+    db = ComponentDatabase.load(tmp_path)
+    assert db.wires == []
+    assert db.capacitors == []