feat: add pcb world with instanced components

Implements Task 6: seeded PRNG for deterministic PCB layout,
substrate, copper trace road, and instanced chip/cap/resistor/via
components. All tests pass. Build successful.

src/App.tsx

@@ -1,5 +1,6 @@
 import { Canvas } from '@react-three/fiber'
 import { CameraRig } from './scene/CameraRig'
+import { World } from './scene/World'
 import { useRideInput } from './ride/useRideInput'
 
 export default function App() {
@@ -15,7 +16,8 @@ export default function App() {
         <ambientLight intensity={0.55} />
         <directionalLight position={[60, 90, 30]} intensity={1.3} />
         <CameraRig />
-        {/* World (Task 6), Bus (Task 7), Sky (Task 8),
+        <World />
+        {/* Bus (Task 7), Sky (Task 8),
             Passengers (Task 7), Stops (Task 9) mount here */}
       </Canvas>
     </div>

src/scene/World.tsx

@@ -0,0 +1,85 @@
+import { useMemo } from 'react'
+import { Instance, Instances } from '@react-three/drei'
+import { TubeGeometry, Vector3 } from 'three'
+import { busPath, pointAt, tangentAt } from './path'
+import { mulberry32 } from './rng'
+
+interface Item {
+  pos: [number, number, number]
+  rot: number
+  scale: [number, number, number]
+}
+
+function layout(seed: number, count: number, minOff: number, maxOff: number): Item[] {
+  const rng = mulberry32(seed)
+  const items: Item[] = []
+  for (let i = 0; i < count; i++) {
+    const t = rng()
+    const side = rng() > 0.5 ? 1 : -1
+    const p = pointAt(t)
+    const tan = tangentAt(t)
+    const off = minOff + rng() * (maxOff - minOff)
+    const n = new Vector3(-tan.z, 0, tan.x).multiplyScalar(side * off)
+    const s = 0.8 + rng() * 2.4
+    items.push({
+      pos: [p.x + n.x, s / 2, p.z + n.z],
+      rot: rng() * Math.PI,
+      scale: [s * (1 + rng()), s, s],
+    })
+  }
+  return items
+}
+
+export function World() {
+  const chips = useMemo(() => layout(1337, 40, 9, 40), [])
+  const caps = useMemo(() => layout(4242, 50, 8, 38), [])
+  const resistors = useMemo(() => layout(90210, 50, 7, 36), [])
+  const vias = useMemo(() => layout(555, 80, 6, 44), [])
+  const trace = useMemo(() => new TubeGeometry(busPath, 200, 0.5, 6, false), [])
+
+  return (
+    <group>
+      {/* substrate */}
+      <mesh rotation={[-Math.PI / 2, 0, 0]} position={[100, -0.05, 0]}>
+        <planeGeometry args={[520, 260]} />
+        <meshStandardMaterial color="#0a3d1e" roughness={0.9} />
+      </mesh>
+      {/* main copper trace = the road */}
+      <mesh geometry={trace} position={[0, 0.02, 0]}>
+        <meshStandardMaterial color="#c47a2c" metalness={0.7} roughness={0.35} emissive="#552f00" />
+      </mesh>
+      {/* chips (black QFP buildings) */}
+      <Instances limit={chips.length}>
+        <boxGeometry />
+        <meshStandardMaterial color="#15151c" roughness={0.6} />
+        {chips.map((it, i) => (
+          <Instance key={i} position={it.pos} rotation={[0, it.rot, 0]} scale={it.scale} />
+        ))}
+      </Instances>
+      {/* electrolytic caps (cylinders) */}
+      <Instances limit={caps.length}>
+        <cylinderGeometry args={[0.6, 0.6, 1, 10]} />
+        <meshStandardMaterial color="#2b3a67" metalness={0.5} roughness={0.4} />
+        {caps.map((it, i) => (
+          <Instance key={i} position={it.pos} scale={[it.scale[1], it.scale[1] * 1.6, it.scale[1]]} />
+        ))}
+      </Instances>
+      {/* resistors (tan boxes) */}
+      <Instances limit={resistors.length}>
+        <boxGeometry args={[2, 0.7, 0.7]} />
+        <meshStandardMaterial color="#c8a165" roughness={0.8} />
+        {resistors.map((it, i) => (
+          <Instance key={i} position={[it.pos[0], 0.35, it.pos[2]]} rotation={[0, it.rot, 0]} scale={it.scale[1]} />
+        ))}
+      </Instances>
+      {/* vias (gold pads) */}
+      <Instances limit={vias.length}>
+        <cylinderGeometry args={[0.5, 0.5, 0.1, 8]} />
+        <meshStandardMaterial color="#e0b341" metalness={0.9} roughness={0.2} emissive="#4a3500" />
+        {vias.map((it, i) => (
+          <Instance key={i} position={[it.pos[0], 0.05, it.pos[2]]} />
+        ))}
+      </Instances>
+    </group>
+  )
+}

src/scene/rng.test.ts

@@ -0,0 +1,13 @@
+import { expect, it } from 'vitest'
+import { mulberry32 } from './rng'
+
+it('is deterministic and in [0,1)', () => {
+  const a = mulberry32(1337)
+  const b = mulberry32(1337)
+  for (let i = 0; i < 100; i++) {
+    const v = a()
+    expect(v).toBe(b())
+    expect(v).toBeGreaterThanOrEqual(0)
+    expect(v).toBeLessThan(1)
+  }
+})

src/scene/rng.ts

@@ -0,0 +1,11 @@
+/** Deterministic PRNG so the PCB layout is stable across builds. */
+export function mulberry32(seed: number): () => number {
+  let a = seed >>> 0
+  return () => {
+    a += 0x6d2b79f5
+    let z = a
+    z = Math.imul(z ^ (z >>> 15), z | 1)
+    z ^= z + Math.imul(z ^ (z >>> 7), z | 61)
+    return ((z ^ (z >>> 14)) >>> 0) / 4294967296
+  }
+}