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/importing-and-picking.md

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+# glTF import and picking
+
+## Import off-thread
+
+The shared importer worker fetches and parses glTF/GLB, then the response hydrates `Mesh` and `PBRMaterial` handles attached to your scene.
+
+```ts
+import { importGltf } from "@yawn/handles";
+
+const meshes = await importGltf(scene, "/models/helmet.glb");
+meshes[0].position[1] = 0.5;
+```
+
+The importer handles triangle primitives, external/data buffers, standard vertex attributes, indices, node transforms, and metallic-roughness values. Application-specific extensions remain application policy.
+
+## Pick in the BVH worker
+
+```ts
+const picking = new Picking(scene);
+await picking.ready;
+
+canvas.addEventListener("click", async () => {
+  const hits = await picking.pick([0, 0, 4], [0, 0, -1]);
+  const nearest = hits[0];
+  if (nearest) console.log(nearest.id, nearest.distance);
+});
+```
+
+The worker reads shared node positions and mesh bounds, updates its BVH when the shared frame counter changes, and returns **all** AABB hits sorted by distance. You can shortlist or run an exact test afterward.
+
+If row allocations relocated since `Picking` was created, refresh its shared descriptors:
+
+```ts
+await picking.refresh();
+```