Fix chamfered crisp edges: anchor the QEM quadric solve at the edge midpoint

[kmatzen]

Problem

After #77, exported meshes show defects along crisp edges: corners visibly chamfered / cut off, even though every vertex sits exactly on the surface and the mesh is watertight.

Root cause is in simplifyMesh's collapse-target solve. Along a straight crease the combined quadric is rank 2 — every point on the crease line has identical (zero) cost — so the raw determinant inverse in optimalQ was ill-conditioned there and let collapse targets slide arbitrarily far along the crease line. Vertices ended up out of order along the edge, and the triangulation chamfered across the corner. The surface-snap pass can't repair it because nothing is off the surface.

Measured on an axis-aligned box at res 128 (max distance from the true crease lines to the mesh, point-to-triangle):

stage	before	after
raw dual contour	0.0000	0.0000
+ error-bounded simplify	1.6398	0.0000
+ surface snap	1.6398	0.0000

The rotated box mostly escaped (0.005) because float noise conditions the system — which is why the existing rotated-corner test didn't catch it.

Fix

Replace the determinant solve with the same truncated-eigen pseudo-inverse the contouring QEF already uses, extracted from qef.ts as solveSymmetric3Anchored, anchored at the edge midpoint: rank-deficient directions keep the midpoint coordinate instead of being dragged by noise, which also preserves vertex ordering along a crease. A guard falls back to endpoint/midpoint candidates if the solved point ever lands far outside the edge's own neighborhood.

Cost: crease-adjacent regions decimate less aggressively (box at res 128: 434 → 1828 triangles). Flat-face decimation is unaffected.

Tests

• New regression test: dual contour → error-bounded simplify on a box, asserting max point-to-triangle distance from all 12 true crease lines stays under half a voxel (was ~1.6 before the fix).
• 258/258 tests pass, tsc --noEmit clean.

🤖 Generated with Claude Code

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