Strip zero-area faces and restitch the T-joints they leave

[revarbat]

Deleting a zero-area face is not enough on its own, which is what made the
last round's diagnosis confusing.

A sliver's three vertices are collinear, so one lies between the other
two. Dropping the face leaves that middle vertex sitting on the interior of
the neighbour's edge — a T-joint — and the two sides no longer share an
edge. That reads as a hole, and is exactly why the checker used to report
722 boundary edges on a mesh that had none.

stripSlivers removes the face and splits the neighbour at the middle
vertex, so the edges match again. No vertex moves: the split point already
lay exactly on the edge it is inserted into. Volume is unchanged, and the
test asserts that rather than trusting the argument.

Needles are a separate case

A zero-area face with two corners at the same position is a needle, not
a T-joint. Its two long edges run between the same pair of points, so once
it is gone the faces on either side already meet — there is nothing to
split, and no middle vertex to split at. It is removed outright and the
coincident pair merged, which makes them share the edge by index as well as
by position.

Two slivers can deadlock

Sharing their long edge, each is the other's only candidate neighbour, so
each kept splitting into the other. A non-sliver neighbour is preferred
now — preferred, not required: refusing sliver neighbours outright made
it far worse (330 → 160), because slivers are commonly adjacent and every
such pair went untouched.

Result

A level-4 Menger sponge built from three intersected extrusions:

Level	Zero-area faces	Manifold	Volume
2	0 → 0	✓	unchanged
3	4 → 0	✓	unchanged
4	330 → 0	✓	drifts 1.6e-07 relative, float32 accumulation over 396,686 triangles

import(repair=true) too

repairMesh dropped faces by repeated index, which disposes of needles —
welding collapses the coincident pair and leaves exactly that — but a
collinear sliver survived it untouched. An imported STL is where those turn
up, so that is where it mattered. The gap was demonstrated before it was
closed: the test failed with "sliver survived repair" while the needle
test beside it passed.

1432 tests pass.

🤖 Generated with Claude Code