Kernel: booleans with coincident planar faces push the faces apart exactly (perpendicular neighbours stretched or shrunk), union cuts the slab beyond the plane

Oracle: 29/29.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/kernel/src/lib.rs

@@ -15,6 +15,7 @@
 mod blend;
 mod body;
 mod build;
+mod coplanar;
 mod heal;
 mod loopblend;
 mod measure;