feat: PhysicsNode + physics-demo sample (roadmap 3.2.0)

[ThomasGorisse]

Summary

Introduces PhysicsNode — a pure-Kotlin rigid-body composable — plus a samples/physics-demo app that lets you tap to throw bouncing balls.

PhysicsNode.kt (sceneview/src/main/java/io/github/sceneview/node/)

Two public types:

PhysicsBody — simulation state for one rigid body:

• Euler integration via Node.onFrame each frame
• Gravity: −9.8 m/s² on Y, dt clamped to 50 ms (absorbs GC spikes)
• Floor collision: position clamp + velocity reflection × restitution
• Sleep: stops integrating when speed falls below 0.05 m/s

PhysicsNode composable — DSL entry point:

PhysicsNode(
    node = sphereNode,
    mass = 1f,
    restitution = 0.6f,
    linearVelocity = Vector3(0f, 5f, 0f),
    floorY = 0f,
    radius = 0.1f
)

samples/physics-demo

• Floor plane (flat CubeNode)
• Tap anywhere → spawns a SphereNode that falls and bounces
• Up to 10 simultaneous balls; oldest removed when cap is hit
• Uses Scene {} (non-AR)

Design decisions

• PhysicsBody is decoupled from Compose — can be stepped from ViewModel or imperative code
• floorY: Float for floor detection — avoids full AABB implementation for this prototype
• mass declared for API completeness / future impulse/force extensions

Test plan

• ./gradlew :samples:physics-demo:assembleDebug
• Tap screen → balls fall and bounce on the floor
• Multiple balls coexist (up to 10)
• Balls come to rest after bouncing settles

🤖 Generated with Claude Code