Improve polygon rendering and benches

[agustin-lowpoly]

Summary

• improve polygon rendering paths, including corner-shape solids, Safari projective fallbacks, and stable same-topology atlas updates
• optimize lossy mesh merging with safer crack guards, bounded merge work, and corpus-backed color/rect-cover heuristics
• clean up the bench directory, move generated bench bundles under .generated, add lossy corpus / trace / voxel report tools, and refresh gallery voxel presets

Validation

• pnpm test
• pnpm build
• pnpm bench:build
• node --check bench/*.mjs
• git diff --check --cached
• lossy targeted risk slice: 332 under-20k source models, 0 count/cost regressions vs baseline