roof(): voronoi method roofs unions of touching pieces

[revarbat]

The default roof() (method="voronoi") gave no roof (and on some inputs allocated without bound) for unions of pieces that touch along edges. On a seeded 6,000-input corpus it failed on 384 before, 16 of them runaways (one reached 36 GB); it now fails on none.

Root causes

1. Union outline breaks Boost.Polygon's input rule. Clipper returns edge-sharing pieces as separate outlines a hairline apart; after snapping, their segments overlap or cross. Boost's Voronoi builder needs segments that meet only at endpoints: overlaps gave a wrong diagram (no roof), crossings never terminated (the runaway). Fix: voronoi now runs the straight method's existing weldContours (weld, re-union, drop straight corners and spikes) first; an outline the weld leaves unchanged except for loop start/order is used as is, so its roof is identical.
2. Boost misbuilds some clean outlines (near-equidistant sites after snapping). Fix: if the mesh is not a valid non-empty solid, retry on grids 2^28 down to 2^25. No previously working input needed a retry.
3. Far unused vertices loosened Manifold's tolerance, so its cleanup collapsed whole roofs or detail (old volumes off by up to 25% vs OpenSCAD). Fix: RoofMesh::build drops unused vertices beyond 16× the mesh extent.

The weld's corner-vs-edge scan now only checks corners in each edge's x range.

Verification

• Corpus (one process per input under a 20 s / 1 GB watchdog): 0 empty/invalid/runaway outputs across triangles, rectangles, rotated, stars, convex, holes, blobs, mixed unions.
• vs OpenSCAD 2026.02.01: closer or tied almost everywhere it gives a roof; 18 cases where OpenSCAD roofs touching pieces separately now differ by 1–14%, and a numerical integral of distance-to-boundary agrees with ours.
• Regressions: previously working inputs keep volume/height/bbox except where the weld changed the outline or the tolerance collapse is fixed; method="straight" is byte-identical on 1,600 inputs.
• Timing: +2–10% on 1000/5000-vertex inputs (the weld's union pass, which straight already pays).
• 5 new gtests in tests/test_extrude_roof.cpp, all failing on the old code. Note: the runaway test allocates without bound on the old code, so beware when bisecting across this change. Full suite: 1468 passed.

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