[#74] Economical-fidelity color default + painterly-grain detail level + postprocess perf

[realproject7]

Fixes #74. Version 0.4.2 → 0.5.0.

What

Makes the default color engine produce faithful yet economical SVGs — keeping the deliberate detail an artist drew (dots, stipple, fine texture) while collapsing only anti-alias/compression fringes — and adds an on-demand max-grain mode for painterly art. Plus a large postprocess speedup that benefits every colour-rich output.

1. Detail-aware region merge (economical fidelity)

In the coverage path a small region merges into its nearest neighbour only when that neighbour is within a ΔE76 threshold (anti-alias / compression noise); a distinct-colour mark farther than it (a dot/stipple/texture drawn on purpose) is kept. One lever delivers both economy and fidelity — no separate node-reduction pass, stipple gate, or complexity estimator.

• preprocess._merge_small_regions(noise_de) + quantize_coverage(region_noise_de=) + PreprocessOptions.coverage_region_noise_de (default 0.0 = legacy merge-everything; fully backward compatible — all other callers/tests unaffected).
• pipeline coverage operating point becomes the validated faithful-economical default; COVERAGE_NOISE_DE maps the existing --detail dial to the threshold.

2. --detail high = painterly brush-grain

At --detail high the coverage path disables the pre-trace flatten and raises its path cap (_COVERAGE_MAX_PATHS_HIGH) so grain-rich output is kept rather than discarded to the baseline path. Lets textured art reach the reference target's grain density on demand (fidelity over economy). A misgated photo still explodes past the high cap and safely falls back.

3. Postprocess perf (every colour-rich output)

_consolidate ran an O(n²) pairwise ΔE scan and recomputed RGB→LAB on every comparison (~30 M calls / ~99 s on a grain-rich trace). Now: short-circuit to identity when palette_threshold <= 0 (the coverage path, where consolidation is a guaranteed no-op) + lru_cache on RGB→LAB. A max-fidelity textured convert: ~110 s → ~12 s, byte-identical output.

Results (vs reference target, 21-image illustration set)

• Bytes 0.2–2.0× reference (was 8–13×); time 6–16 s (was up to 218 s); 14/21 ≥ 0.90 SSIM-to-reference; detail (dots/stipple/texture/outlines) visually preserved.
• Heavily-textured outlier at --detail high: full grain density (≈ reference) in ~12 s.

Safety / no-regression

• Default (normal) color output byte-identical where unchanged — corpus goldens green.
• Flat/simple art unaffected (Priority EPIC: svgsmith — agent-native self-verifying raster→SVG vectorizer #1); gradients unaffected (smooth = all low-ΔE, gate never fires); photos protected by the coverage path-count fallback.
• Full suite green except the known env-only test_version_is_single_sourced (editable-metadata artifact; passes in CI). ruff check src tests clean.
• New tests: test_merge_small_regions_noise_de_keeps_distinct_marks, test_coverage_noise_de_dial_orders_detail_levels, test_palette_threshold_zero_merges_nothing.

dist built + twine check PASSED (svgsmith-0.5.0).