fix: deterministic physical item motion in 3D demo

Add physicalItemMotion as the single source of item pose, synchronize belt/item speed at 1 m/s, route items through chutes, keep C/D items settled inside roll-cages, render historical settled items, and use STL models with explicit fallbacks.

src/domain/physicalItemMotion.test.ts

@@ -0,0 +1,47 @@
+import { describe, it, expect } from 'vitest';
+import { getPhysicalItemPose } from './physicalItemMotion';
+import { ZONES, BELT_TOP_Y, CONVEYOR_SPEED_MPS } from './physicalLayout';
+
+describe('physicalItemMotion', () => {
+  const defaultInput = {
+    caseId: 'test_case',
+    dimensionsMm: { width: 300, depth: 200, height: 200 }, // 0.2m height
+    targetCategory: 'B' as any,
+    elapsedMs: 0,
+    slotIndex: 0
+  };
+
+  it('starts at spawn point on the belt', () => {
+    const pose = getPhysicalItemPose({ ...defaultInput, elapsedMs: 0 });
+    expect(pose.surface).toBe('belt');
+    expect(pose.position[0]).toBe(ZONES.A.x);
+    expect(pose.position[1]).toBe(BELT_TOP_Y + 0.1); // 0.7 + 0.1
+    expect(pose.phase).toBe('feed');
+  });
+
+  it('moves at 1 m/s along the belt during move_to_detection', () => {
+    // move_to_detection starts at 300ms
+    const pose = getPhysicalItemPose({ ...defaultInput, elapsedMs: 1300 });
+    const distance = 1000 / 1000 * CONVEYOR_SPEED_MPS; // 1.0 m
+    expect(pose.position[0]).toBeCloseTo(ZONES.A.x + distance, 2);
+    expect(pose.surface).toBe('belt');
+  });
+
+  it('stays in C cage for category C after routing is complete', () => {
+    // 9800ms is the total duration
+    const pose = getPhysicalItemPose({ ...defaultInput, targetCategory: 'C', elapsedMs: 10000 });
+    expect(pose.isSettled).toBe(true);
+    expect(pose.surface).toBe('c_cage');
+    expect(pose.position[0]).toBeCloseTo(ZONES.C.x - 0.2, 1);
+    expect(pose.position[2]).toBeCloseTo(ZONES.C.z - 0.1, 1);
+    expect(pose.position[1]).toBeCloseTo(0.1 + 0.1, 2); // floor + half height
+  });
+
+  it('stays in D cage for category D after routing is complete', () => {
+    const pose = getPhysicalItemPose({ ...defaultInput, targetCategory: 'D', elapsedMs: 10000 });
+    expect(pose.isSettled).toBe(true);
+    expect(pose.surface).toBe('d_cage');
+    expect(pose.position[0]).toBeCloseTo(ZONES.D.x - 0.2, 1);
+    expect(pose.position[2]).toBeCloseTo(ZONES.D.z - 0.1, 1);
+  });
+});