Add tutorial docs and interactive playgrounds

Amp-Thread-ID: https://ampcode.com/threads/T-01a01380-b478-77d0-84a0-102880a5c5ae
Co-authored-by: Heaust Azure <heaust.azure@gmail.com>

docs/packages/mesh-handles.md

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+# Conventional handles
+
+`@yawn/mesh-handles` is an optional object-oriented facade. It never hides core: lifecycle methods call core commands, while frequent mutations write shared rows.
+
+## Meshes and instances
+
+Wrap imported descriptors, use the default instance created by glTF, or create another generation-safe instance.
+
+```js
+const handles = new MeshHandles(core);
+const [mesh] = handles.fromImportedScene(imported);
+
+mesh.defaultInstance.setTransform(matrix);
+const duplicate = await mesh.createInstance(otherMatrix);
+duplicate.setType(classificationWords);
+await duplicate.destroy();
+```
+
+The handle is `[slot, generation]`. A stale object cannot modify a slot that has since been reused.
+
+## Camera and material handles
+
+The camera and materials look conventional, but property updates are writes to `camera.state` and `material.state`.
+
+```js
+const camera = new CameraHandle(core);
+camera.lookAt([4, 3, 6], [0, 0, 0]);
+
+const materials = new MaterialHandles(core).fromImportedScene(imported);
+materials[0].baseColor = [0.2, 0.55, 1, 1];
+materials[0].roughness = 0.35;
+```
+
+<Playground
+  id="conventional-handles"
+  title="Camera and material properties"
+  description="The gallery is reframed and one PBR row is restyled with direct shared-memory writes."
+/>
+
+## Worker-side picking
+
+Picking is lazy. The first `pickRay` starts a separate spatial-query worker over versioned render-data snapshots and returns wrapped instance handles.
+
+```js
+const result = await handles.pickRay(origin, direction, {
+  maxDistance: 10_000,
+  maxHits: 1,
+});
+
+const picked = result.hits[0]?.instance;
+```
+
+<Playground
+  id="picking"
+  title="Pick the closest shared instance"
+  description="Build the optional BVH and issue a ray query without adding picking code to core."
+/>
+
+Call `handles.dispose()` when the scene ends so its optional picking worker and listeners are released.