perf: improve GSplat compute renderer cold-start buffer capacities

[mvaligursky]

Raises initial buffer capacities in the compute GSplat renderer to eliminate missing tiles / dropped large splats in the first few frames after scene load or camera teleport, before GPU readback has converged.

Changes:

• INITIAL_TILE_ENTRY_MULTIPLIER: 1.5 → 2.5 — floor on tile-entry capacity, ensures frame-1 buffer is large enough for dense scenes.
• ENTRY_HEADROOM_MULTIPLIER: 1.5 → 2.0 — cushions steady-state frame-to-frame spikes (camera motion, splats crossing tile boundaries).
• INITIAL_LARGE_SPLAT_CAPACITY: 4096 → 16384 — avoids dropping large splats on close-up views before the grow-only readback path catches up.
• Restructure the module-top constants block into three clearly-labeled groups (tunable knobs, algorithmic invariants, module-scope scratch) with expanded comments documenting the trade-off and memory cost of each knob.

Rationale:
The compute GSplat renderer targets desktop platforms, so the small additional memory cost is worthwhile in exchange for eliminating visible cold-start artifacts. Worst-case additional memory is dominated by the tile-entry buffers (numSplats × multiplier × 8 bytes); for a 10M-splat scene this is on the order of tens of MB and occurs only when needed.

No public API change.