refactor(gsplat): extract GSplatWorld from GSplatManager

[mvaligursky]

Splits GSplatManager's two unrelated responsibilities apart, as the first step toward letting the hybrid GPU-sort renderer share splat data across multiple renderers (e.g. a future shadow renderer).

Changes:

• New internal GSplatWorld (src/scene/gsplat-unified/gsplat-world.js) owns the work buffer, versioned world states, allocation, octree/LOD evaluation, streaming, budget enforcement, and the work-buffer bake.
• GSplatManager keeps the single renderer, the sorters, interval compaction, the projector, and per-camera cull/sort — and delegates the world responsibilities to its GSplatWorld.
• The dependency is strictly one-way (manager → world): the manager reads world data through getters and drives mutation via explicit methods (update/bake/markSorted/onSorted/syncFormat/invalidate/reconcile). Results are written into caller-owned persistent objects to avoid per-frame allocation.
• 1:1 ownership for now (each manager creates one world, no sharing yet); behaviour-preserving.
• GSplatIntervalCompaction.invalidateUpload() added to replace a direct private-field poke from the manager.
• LOD-cadence fix: the periodic (10-frame) LOD update now uses a latched request, so an update deferred by sorter back-pressure fires on the next available frame instead of being lost until the next cadence interval.

Performance:

• No expected change for the steady state. The LOD-cadence latch makes a back-pressure-deferred LOD update fire up to ~10 frames sooner under load.

All GSplatWorld / GSplatManager members are internal (@ignore); no public API changes.