feat: Add Scene#gsplatCentersEnabled opt-out for gsplat CPU centers

[mvaligursky]

Adds an opt-out so loaded gsplat assets can omit the CPU-side centers buffer when only unified GPU sorting (or other paths that do not need it) is required, reducing memory for large SOG/PLY loads.

Changes:

• New Scene#gsplatCentersEnabled (default true) with JSDoc; each parser load snapshots the flag before its first await and passes prepareCenters into resources.
• GSplatResourceBase: _centers storage with centers accessor, optional prepareCenters in constructor, and hasCenters (non-null check without triggering container lazy centers).
• File-backed resources forward options; SOG parsers skip prepareGpuData when centers are disabled at load.
• GSplatContainer: lazy centers allocation via super(..., { prepareCenters: false }).
• Unified GSplatManager: when CPU sorting is active, omit placements without centers; _worldStateDirty on renderer mode change; clearer warnOnce messages.
• Non-unified GSplatInstance: gate on hasCenters, matching warning style.

Hidden API:

• Scene#gsplatCentersEnabled — boolean, default true. Set to false before starting a gsplat asset load to omit centers data for that load (see JSDoc for GSPLAT_RENDERER_RASTER_CPU_SORT and timing).

Impact

• this saves around 114MB for 10M splats.