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/ops.rs

@@ -549,6 +549,14 @@ fn boolean_whole(a: &Body, b: &Body, op: BoolOp) -> Result<Option<Body>> {
             return Ok(Some(body));
         }
     }
+    // Faces back to back over part of their area: imprinted on each other and glued (exact; held
+    // to the retry guard's checks all the same).
+    if op == BoolOp::Union && crate::coplanar::faces_touch(a, b) {
+        let accept = |body: &Body| check(body).0 && !unfused(body) && crate::boolean_guard::check_retry(a, b, op, Some(body)).is_ok();
+        if let Some(body) = crate::coplanar::imprint_union(a, b, &accept) {
+            return Ok(Some(body));
+        }
+    }
     let mut empty_votes = 0;
     let mut empty_diagnostics = crate::BooleanDiagnostics::default();
     let keep: Vec<Vec<Vec3>> = a.split_keep().iter().chain(b.split_keep()).cloned().collect();