fix(kernel): preserve intentional Split Face lines in contact joins (#782)

Intentional Split Face lines survive a join with partial planar contact, so the far face stays in two pieces. Follow-up to #704 / #674.

crates/kernel/src/lib.rs

@@ -40,6 +40,8 @@ mod intersection_curve_tests;
 #[cfg(test)]
 mod issue_616_tests;
 #[cfg(test)]
+mod issue_674_tests;
+#[cfg(test)]
 mod issue_679_tests;
 mod lip;
 mod loopblend;