Chamfer: refuse a result that grows a convex edge's part (#1428)

* Chamfer: refuse a result that grew a convex edge's part (or shrank a concave one's)

A chamfer of a curved edge that runs off a face it skims (robustness seed 57's
ball rim) built a part bigger than before. Checked from the two faces' normals
at the edge; rounds and unclear angles are left to the wedge bound.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Action matrix: the sheet metal tray's relief corners refuse a 0.25 mm chamfer

Each is a 2.5 mm concave edge (the corner of a corner relief, between two
planes at right angles): a chamfer there fills at most 0.25²/2 × 2.5 = 0.08
mm³. What was built took 0.95 mm³ away; the chamfer guard now refuses it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Robustness goldens: seed 57's wrong-way chamfer is now refused (#1390)

On main with #391, seed 57's chamfer grew the part; this refuses it, so the run goes on
to its later steps and ends ok.

---------

Co-authored-by: Phanoob <phanoob@gmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: bherbruck <brennen.herbruck@gmail.com>

crates/kernel/src/lib.rs

@@ -17,6 +17,8 @@ mod blend_corner;
 mod blend_fit;
 mod blend_variable;
 mod blend_vertex;
+#[cfg(test)]
+mod blend_wrong_way_tests;
 mod body;
 mod boolean_diagnostics;
 mod boolean_guard;