fix: keep seam overlap repairs convex

[agustin-littlehat]

Summary

• reject seam overlap repairs that would turn a render polygon concave
• keep the original face when the repaired outline is not weakly convex
• add a castle regression test for lossy seam overlap output

Validation

• pnpm --filter @layoutit/polycss-core test -- src/merge/optimizePolygons.test.ts
• git diff --check origin/main..HEAD
• node bench/lossy-corpus-bench.mjs --models obj/castle.obj --quick --json /tmp/polycss-castle-convex-branch.json --limit 10