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/materials.md

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+# Materials and textures
+
+`PBRMaterial` is a conventional view over `materials` and `materialTextures` SOA rows.
+
+```ts
+const paint = new PBRMaterial(scene, {
+  baseColor: [0.8, 0.05, 0.03, 1],
+  metallic: 0.65,
+  roughness: 0.22,
+  emissive: [0, 0, 0],
+});
+await paint.ready;
+
+paint.roughness = 0.5;       // direct SAB write
+paint.baseColor[1] = 0.35;   // direct SAB write
+mesh.material = paint;
+```
+
+<Playground example="materials" />
+
+## Graph textures
+
+```ts
+const albedo = new Texture(scene, {
+  source: "/textures/paint.ktx2",
+  size: [2048, 2048, 1],
+  format: "rgba8unorm-srgb",
+  usage: ["sampled", "copyDst"],
+});
+await albedo.ready;
+
+const textured = new PBRMaterial(scene, { baseColorTexture: albedo });
+```
+
+Creating or removing a `Texture` rebuilds the single graph loadout so the GPU resource is allocated up front. The source pointer stays on the handle for an importer or application uploader; core never owns image-loading policy.
+
+## Custom WGSL
+
+`ShaderMaterial` adds its external WGSL pipeline to the scene graph. Updating the code updates the loadout.
+
+```ts
+const shader = new ShaderMaterial(scene, {
+  code: `
+    struct Out { @builtin(position) position: vec4<f32> }
+    @vertex fn vertex(@builtin(vertex_index) id: u32) -> Out {
+      let p = array(vec2(-.2, -.2), vec2(.2, -.2), vec2(0., .2));
+      var out: Out; out.position = vec4(p[id], 0., 1.); return out;
+    }
+    @fragment fn fragment() -> @location(0) vec4<f32> {
+      return vec4(1., .2, .7, 1.);
+    }
+  `,
+});
+await shader.ready;
+```
+
+<script setup>
+import Playground from "../.vitepress/Playground.vue";
+</script>