Make seam repair the default

[agustin-littlehat]

Summary

• move seam repair into core and run bleed/split by default on the lossy mesh path
• remove seam bleed/split/show mesh workbench controls while keeping interior fill as a separate off-by-default experiment
• sync React, Vue, vanilla, docs, and the seam gap bench with the new default path

Verification

• pnpm test
• pnpm build:packages
• pnpm build:website
• node --check bench/seam-gap-bench.mjs
• git diff --check
• Playwright smoke check for gallery rendering controls