Replace addons with conventional handles

Provide the single-loadout Scene API, SAB-backed meshes, cameras, materials, lights, workers, post effects, and tutorial playgrounds. Batch matching row growth so active GPU loadouts refresh once.

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

docs/guide/meshes-and-instances.md

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+# Meshes and instances
+
+Every `Mesh` is an instance. `clone()` shares geometry and allocates only a new node/mesh slot.
+
+```ts
+const source = new Mesh(scene, {
+  vertexData: {
+    positions: [-0.2, -0.2, 0, 0.2, -0.2, 0, 0, 0.25, 0],
+    indices: [0, 1, 2],
+  },
+});
+await source.ready;
+
+for (let x = -4; x <= 4; x++) {
+  const instance = source.clone({ position: [x * 0.2, 0, 0] });
+  await instance.ready;
+}
+```
+
+<Playground example="instances" />
+
+## Copy-on-write geometry
+
+The default vertex kinds are `positions`, `normals`, `tangents`, `uvs`, `colors`, and `indices`. Mutating any kind on an instanced clone first makes that mesh's geometry unique.
+
+```ts
+const clone = source.clone();
+await clone.ready;
+
+await clone.setVertexData("positions", [
+  -0.4, -0.2, 0,
+   0.4, -0.2, 0,
+   0.0,  0.5, 0,
+]);
+```
+
+## Per-face materials and visibility
+
+```ts
+await mesh.setMaterialForFaces(red, [0, 2, 4]);
+mesh.material = blue;
+mesh.isVisible = false; // one direct u32 write
+```
+
+Face rows store optional material pointers; a zero lane falls back to `mesh.material`.
+
+<script setup>
+import Playground from "../.vitepress/Playground.vue";
+</script>