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/engine/src/tests/robustness.rs

@@ -202,8 +202,8 @@ fn fillet_into_an_earlier_round() {
     }
 }
 
-/// Clean seeds, and the original seed 5 edge contact and seed 16 partial face contact, now
-/// rejected without distortion.
+/// Clean seeds (seed 16: a partial face contact, fused), and the original seed 5 edge contact,
+/// now rejected without distortion.
 #[test]
 fn fixed_seeds_run_clean() {
     for seed in FIXED_SEEDS {
@@ -228,12 +228,10 @@ fn fixed_seeds_run_clean() {
             continue;
         }
         if *seed == 16 {
-            // Seed 16 patterns a short cylinder around a block: one copy's cap lies on a side of
-            // the block but half of it runs past the block's bottom edge. That contact can't be
-            // fused yet (#674). Before #577 the join came back as two shells touching over the
-            // half disc (#396); with #577 alone, the cap was pushed into the block over its whole
-            // area, which added 72 mm³ past the edge (45290.6 mm³ for 45218.4). The retry guard (#644) refuses that,
-            // so the pattern is refused and the part stays as it was.
+            // Seed 16 patterns a short cylinder around a block: one copy's end lies on a side of
+            // the block with half of it past the block's bottom edge (#674). The end and the side
+            // are split along each other's outline and the half disc they share goes: the block,
+            // half of the first cylinder (the other half is inside the block) and three whole ones.
             let mut s = Session::default();
             run(&mut s, "solid.box", json!({"length": 25.0, "width": 47.5, "height": 37.5}));
             run(
@@ -241,13 +239,13 @@ fn fixed_seeds_run_clean() {
                 "solid.cylinder",
                 json!({"axis": [1.0, 0.0, 0.0], "base": [0.0, 7.5, 0.0], "height": 2.5, "operation": "join", "radius": 5.0}),
             );
-            let before = total_volume(&s);
-            // The block, and the half of the cylinder outside it.
-            assert!((before - (25.0 * 47.5 * 37.5 + PI * 25.0 * 2.5 / 2.0)).abs() < 0.05, "{before}");
-            let error = s.execute("solid.pattern.circular", &json!({"axis": "Z", "count": 4, "features": ["Cylinder1"]})).unwrap_err();
-            assert!(error.to_string().contains("touch along faces that could not be fused"), "{error}");
-            assert!((total_volume(&s) - before).abs() < 1e-6);
+            run(&mut s, "solid.pattern.circular", json!({"axis": "Z", "count": 4, "features": ["Cylinder1"]}));
+            // (Within the measured round sides' mesh error, about 0.03 mm³ a cylinder.)
+            let want = 25.0 * 47.5 * 37.5 + 3.5 * PI * 25.0 * 2.5;
+            assert!((total_volume(&s) - want).abs() < 0.2, "{} vs {want}", total_volume(&s));
             assert!(s.model.state().bodies.iter().all(|b| b.body.validity().is_empty()));
+            let r = fuzz::run(*seed, 8);
+            assert!(!r.outcome.is_bug(), "seed {seed}: {:?}\n{}", r.outcome, r.script());
             continue;
         }
         let r = fuzz::run(*seed, 8);