Renderer: add reference-backed parity scenes for compatibility toggles

[akratch]

Problem\nThe renderer is playable, but several compatibility defaults are intentionally approximate: prop-attached bullet impacts, per-room scissor boxes, sky fallback, and a frontend/menu brightness residual. These are documented, but each needs a focused local scene and pass/fail signal before the exact paths can be promoted.\n\n## Desired end state\n- Each renderer compatibility toggle has a small reproducible scene.\n- The local capture commands produce screenshots/traces under ignored paths only.\n- compare_screenshots.py or a successor can support repeatable A/B checks against a local reference.\n\n## Candidate toggles\n- GE007_TEXTURED_PROP_BULLET_IMPACTS=1\n- GE007_EXACT_ROOM_SCISSOR=1\n- GE007_PORTAL_BFS=0\n- GE007_DIAG_SETTEX_CC_COLOR_SCALE\n\n## Validation\n- ./tools/renderer_parity_capture.sh --no-build\n- ./tools/validate_quick.sh\n- ./scripts/ci/check_release_ready.sh\n\n## Asset rule\nDo not commit or attach captured screenshots, textures, or ROM-derived visual data.