v0.6: Actor system (rigidbody) replaces pixel player

- New actors.rs: Rigidbody Actor with gravity, grid collision, powder displacement
- Actor rendered as colored rectangle on pixel buffer (zoom-scaled)
- WASM API: spawn_actor, move_actor, update_actor, actor_x/y
- WASD/Arrow keys control actor, camera follows with soft lerp
- displace_powders pushes sand/dirt out of actor area
- Collision: stops at solids, walks through powders
- Updated AGENTS.md with actor documentation

web/src/main.ts

@@ -99,9 +99,9 @@ async function main() {
     }
   }
 
-  // === Spawn point: center on the castle ===
+  engine.spawnActor(400, 550);
   camera.x = 400;
-  camera.y = 630;
+  camera.y = 550;
 
   const rw = engine.renderWidth();
   const rh = engine.renderHeight();
@@ -184,11 +184,21 @@ async function main() {
       );
     }
 
-    const speed = 200;
-    if (input.isDown("KeyA") || input.isDown("ArrowLeft")) camera.x -= speed * dt;
-    if (input.isDown("KeyD") || input.isDown("ArrowRight")) camera.x += speed * dt;
-    if (input.isDown("KeyW") || input.isDown("ArrowUp")) camera.y -= speed * dt;
-    if (input.isDown("KeyS") || input.isDown("ArrowDown")) camera.y += speed * dt;
+    let mx = 0;
+    let my = 0;
+    if (input.isDown("KeyA") || input.isDown("ArrowLeft")) mx = -1;
+    if (input.isDown("KeyD") || input.isDown("ArrowRight")) mx = 1;
+    if (input.isDown("KeyW") || input.isDown("ArrowUp")) my = -1;
+    engine.moveActor(mx, my);
+    engine.updateActor(dt);
+
+    const ax = engine.actorX();
+    const ay = engine.actorY();
+    if (ax > 0 || ay > 0) {
+      camera.x += (ax - camera.x) * 10 * dt;
+      camera.y += (ay - 60 - camera.y) * 10 * dt;
+    }
+
     if (input.isDown("Equal")) camera.setZoom(camera.zoom + 2 * dt);
     if (input.isDown("Minus")) camera.setZoom(camera.zoom - 2 * dt);
     if (input.wheel !== 0) {