fix: fuse bodies that touch back to back over part of a face (#679, #674) (#704)

Two L's extruded up and down from one 2 x 2 grid now join into one solid (the planar rebuild gives each fan of faces around a pinched vertex its own vertex copy), and a pad whose end lies half past the edge of the face it stands on fuses exactly: contact faces are imprinted with each other's outline and the common faces glued away, still held to the retry guard. Face splitting handles a line crossing an edge twice; glue matches differently parametrised curves and keeps several pieces apart. Robustness seed 16 now fuses with the exact volume.

Fixes #679, fixes #674.

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

crates/kernel/src/lib.rs

@@ -39,6 +39,8 @@ mod iges_out;
 mod intersection_curve_tests;
 #[cfg(test)]
 mod issue_616_tests;
+#[cfg(test)]
+mod issue_679_tests;
 mod lip;
 mod loopblend;
 mod measure;