Improve solid seam coverage and interior fill

[agustin-littlehat]

Summary

• expand solid CSS primitives slightly to reduce seams across border-shape, triangle, and projective quad paths
• replace the gallery interior fill implementation with appended oval cavity fills, enabled by default behind the existing checkbox
• remove the gallery overpaint KPI plumbing

Validation

• pnpm test && pnpm build
• gallery smoke: interior fill checkbox defaults on, no margin control, oval fill leaves append at the end of the DOM mesh