feat(bench): add 4 SplatBench corpus scenes (specular, foliage, lowlight, portrait)

Extend the SplatBench v0 corpus from 7 to 11 scenes with synthetic probes
targeting specific Gaussian-splat failure modes:

- splatbench_specular_proxy   (12k splats)  view-dep SH cluster; stresses
                                            SH-degree reduction
- splatbench_foliage_proxy    (30k splats)  thin anisotropic + mid-opacity;
                                            stresses opacity-prune + sort
- splatbench_lowlight_proxy   (8k splats)   dark DC + high-magnitude SH;
                                            stresses u8 color quantization
- splatbench_portrait_proxy   (12k splats)  dense face + sparse wall;
                                            stresses saliency-aware pruning

Each scene is deterministic (per-seed) and small enough to commit as a
fixture. Adds them to synth_scenes.py, the fidelity runner's scene order,
and re-runs analyze + optimize + fidelity to emit hashes, SPZ ratios, and
visual-fidelity metrics for the published leaderboard.

All 11 scenes round-trip cleanly through the renderer (pass under both
web-mobile and size-min presets). Pre-existing 7 scenes' SPZ ratios are
unchanged.

Author: montabano1

.gitignore

@@ -25,6 +25,16 @@ tests/visual/.cache/
 /benches/scratch/
 /benches/runs/
 
+# Pre-v0.1.2 SplatBench scenes regenerate via `python3 benches/synth_scenes.py
+# benches/scenes`; they're large (the dense proxy is 473 MB) and not committed.
+# The v0.1.2 failure-mode scenes are small enough to ship as fixtures and ARE
+# committed under benches/scenes/.
+/benches/scenes/splatbench_product_proxy.ply
+/benches/scenes/splatbench_indoor_proxy.ply
+/benches/scenes/splatbench_outdoor_proxy.ply
+/benches/scenes/splatbench_floater_proxy.ply
+/benches/scenes/splatbench_dense_proxy.ply
+
 # Editor / OS
 .DS_Store
 *.swp