handles: one interleaved vertices/indices arena

Geometry becomes a span in a single 80-byte interleaved `vertices` array
plus a span in a single `indices` array, allocated through the core's
span allocator instead of one arena array per mesh attribute. Draws are
always indexed with baseVertex; face-material overrides emit one draw per
run of same-material faces rather than one per face. Vertex data is
validated as a whole before any span is touched, same-geometry mutations
are serialised, and render signal 4 is held across a span change until
the graph reflecting it is published. Sibling arrays grow to the primary
array's capacity in one batch; reserve() takes vertex/index totals.

Amp-Thread-ID: https://ampcode.com/threads/T-01a0b2dd-db32-712d-99ce-12ce4a86d979
Co-authored-by: Heaust Azure <heaust.azure@gmail.com>

addons/handles/src/importers/gltf.ts

@@ -45,6 +45,20 @@ export async function importGltf(scene: Scene, url: string | URL) {
   await scene.reserve({
     nodes: result.primitives.length,
     materials: result.materials.length,
+    vertices: result.primitives.reduce(
+      (total: number, primitive: any) => total + primitive.positions.length / 3,
+      0,
+    ),
+    // Each geometry's index span is padded to whole rows of four.
+    indices: result.primitives.reduce(
+      (total: number, primitive: any) =>
+        total +
+        Math.ceil(
+          (primitive.indices?.length ?? primitive.positions.length / 3) / 4,
+        ) *
+          4,
+      0,
+    ),
   });
   let textures: Texture[] = [];
   const imported = await scene.batchGraphUpdates(async () => {