Mesh pages stay under 128 KiB and each region page names the mesh pages it needs

[pasquelin]

Why

Split from #762 (~900 lines): after #762 moves clusters.json out of the root into mesh pages read through a tree, the mesh pages must stay small and the region pages must name them, so the runtime (#751) fetches only what a region needs.

To do

• Mesh pages under 128 KiB: the writer cuts mesh pages under 128 KiB each.
• Region pages name their mesh pages: each region page of The compiler writes the manifest in pages a reader fetches by region #750 lists the mesh pages its cells use.

Code context

#762's tree and readers, #781's region pager (generalised, never copied), docs/FORMAT.md.

Proof

Goldens updated with the format bump; every mesh page ≤ 128 KiB on the open world's cook; a region page lists exactly the mesh pages its cells use.

Links

Parent #483. Blocked by #762. Split from #762.

[pasquelin]

taken by lead compiler. Design note: the manifest writer of #762 (compiler_manifest_pages.rs, one mesh page today) cuts mesh pages under 128 KiB through the shared pager of #796/#781 (generalised, never copied), filling the root's eight slots and index pages beyond; each region page of #750 gains the list of mesh-page slots its cells' primitives live in. Budget: every mesh page ≤ 128 KiB (PAGE_BYTES), root size fixed. Paths: cook and format only (CPU); no WebGPU/WebGL2 draw change, the runtime fetch by region is #751. Scenes: grid(48)/grid(192) fixtures (cut and listing tests) and the open world's cook (every mesh page ≤ 128 KiB). Format bump with goldens.