RFC §unification: audit asset inference completeness + spec AssetManifest lifetime

[apotema]

Part of #560.

Two follow-ups to the asset-inference ticket:

1. Audit completeness. Walk in-tree projects (flying-platform-labelle and others) for atlases or audio banks loaded only from scripts — i.e., never reachable by Sprite-ref inference. Document which scenes/prefabs will need an explicit AssetManifest post-migration so we don't silently drop assets when the "assets" field is removed.

2. AssetManifest lifetime / refcount. Spec what happens when a prefab carrying AssetManifest is destroyed:

   • Reference-counted per bundle?
   • Multiple instantiations of the same prefab — single load + single unload, or per-instance refcount?
   • Bundles referenced by multiple manifests — unload only when the last manifest is gone?

Default lean: per-bundle global refcount; load on first manifest, unload when refcount hits zero.

[apotema]

RFC updated with `AssetManifest` lifetime semantics — see the "Assets — inference + lazy fallback over `AssetCatalog`" section in `RFC-UNIFY-SCENES-AND-PREFABS.md`.

Resolved:

• `AssetManifest` declarations are static prefab data. The walker (RFC §unification: sprite-based asset inference + AssetManifest component #563) unions them into the prefab's pre-computed `required_resources` set at engine startup.
• Manifest-declared resources are acquired at top-level spawn and released at root destruction — same lifecycle as inference-derived resources.
• Refcount is handled by `AssetCatalog` (already implemented). Multiple instantiations of the same prefab carrying the same manifest each bump refcount; unload only when the count hits zero.

Remaining scope for this ticket:

• Completeness audit. Walk in-tree projects (flying-platform-labelle and others) to identify atlases / audio banks loaded only from scripts — i.e., references the walker can't see statically. Document which scenes/prefabs will need an explicit `AssetManifest` post-migration so we don't silently drop assets when `"assets"` is removed.
• Cross-check: the lazy-on-miss fallback (RFC §unification: sprite-based asset inference + AssetManifest component #563) catches sprite/image references at runtime. The audit's purpose is to find cases where lazy fallback isn't enough — e.g., audio banks where the sound has to be ready before the trigger fires, or sprites that must be visible from frame 1.