Illustration-geometry knobs (supersample + dark-thin) + quality harness — loop-ready, default-off

[realproject7]

Phase 0 of the daily quality loop (see ~/Projects/docs/MANUAL-svgsmith-quality-loop.md): make the engine loop-ready by exposing the two confirmed illustration-geometry levers as tunable, default-OFF knobs, plus a reusable comparison harness.

No-regression by construction

Both knobs default to 0 (off) → behaviour unchanged. Verified byte-identical to 0.4.1 (md5) on pigeon / logo / 16-gradient / 12-sticker. The knobs apply ONLY to the outlined low-res illustration class on the coverage path (gated by _supersample_candidate); gradients, photos, and high-res inputs ignore them.

Knobs

• ConvertOptions.illustration_supersample — trace the flat-colour mask at this resolution (e.g. 2048) → round, uniform feather/scallop boundaries (SSIM-to-reference 0.896 → ~0.93).
• ConvertOptions.illustration_dark_thin — erode the dark linework by N steps → thick outline blobs become thin crescents; eye pupils / beak tip protected by area. (preprocess._thin_dark + quantize_coverage(dark_thin=).)
• Both validated (non-negative); a regression test asserts they're opt-in (default native viewBox; illustration_supersample=2048 → larger internal resolution).

Harness

tools/quality_compare.py <folder> [--supersample N] [--dark-thin N] — given <folder>/inputs + <folder>/ref-svg, runs svgsmith with the chosen knobs → ours/, builds input | ours | reference target montages → montage/, writes metrics.md (SSIM to the reference OUTPUT, not the original — the correct loop metric — plus palette/paths/bytes). Outside src/, not packaged in the wheel.

Known follow-ups (deliberately deferred to the loop's R&D — not stubbed)

• The dark-thin nearest-colour fill can leave faint grey; high-res tracing inflates node count. A node-economical curve-fit / simplify pass is the next lever — the loop's core R&D target.

Full suite green; ruff clean (ruff check .). Version 0.4.1 → 0.4.2.