mesh: an enclosure's corner bosses are ringed where its walls join them (#402)

A boss the body's walls run into had its circle crossed by the walls' planes, so the projection
layout refused it and the plan fell back to swept cells, fanning round the boss and its hole.
Such a boss is now framed by the square round its whole circle, subdivided by the planes that
cross it; every square inside (the hole's, the core) is divided alike, so the ring's blocks meet
the walls node to node. On the walls' side the frame stops at their inner plane and its corner
projects onto the middle of the inner fillet, which the ring takes in (a boss just touching the
fillet included: no sliver). Cylinders a fillet's history names but that are a ring's circle are
the ring's faces. Rings are built per patch of grid cells, the one-cell O-grid being the
simplest patch.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/cmd/plan.rs

@@ -1559,6 +1559,35 @@ mod acceptance {
         accept(&mut s, &r, 0.6, 6.0);
     }
 
+    /// An enclosure base: a box with rounded vertical edges, shelled open at the top, a screw
+    /// boss joined at each corner (centred outside it, nearly touching the inner fillet) with a
+    /// blind hole, and standoffs on the floor. Each boss's ring frames the whole circle,
+    /// subdivided where the walls join it; the inner fillet is taken in by the ring's corner and
+    /// the boss's near touch with it leaves no sliver.
+    #[test]
+    fn an_enclosure_base_rings_each_corner_boss_where_the_walls_join_it() {
+        let mut commands = vec![
+            json!({"command": "solid.box", "params": {"length": 56, "width": 42, "height": 16, "body_name": "Base"}}),
+            json!({"command": "solid.fillet", "params": {"edges": [[0, 0, 8], [56, 0, 8], [56, 42, 8], [0, 42, 8]], "radius": 5}}),
+            json!({"command": "solid.shell", "params": {"faces": [[28, 21, 16]], "thickness": 3, "body": "Base"}}),
+        ];
+        for (x, y) in [(-1.0, -1.0), (57.0, -1.0), (-1.0, 43.0), (57.0, 43.0)] {
+            commands.push(json!({"command": "solid.cylinder", "params": {"diameter": 13, "height": 16, "base": [x, y, 0], "operation": "join", "targets": ["Base"]}}));
+        }
+        for (x, y) in [(-1.0, -1.0), (57.0, -1.0), (-1.0, 43.0), (57.0, 43.0)] {
+            commands.push(json!({"command": "solid.hole", "params": {"position": [x, y, 16], "diameter": 3.3, "depth": 8}}));
+        }
+        for (x, y) in [(16.5, 16.5), (39.5, 16.5)] {
+            commands.push(json!({"command": "solid.cylinder", "params": {"diameter": 6, "height": 4, "base": [x, y, 3], "operation": "join", "targets": ["Base"]}}));
+        }
+        let (_d, mut s) = open(json!(commands));
+        let plan = s.execute("mesh.plan", &json!({"body": "Base", "size": 1.5, "layers": 2})).unwrap();
+        assert_eq!(plan["mesher"], "project", "{plan}");
+        let r = s.execute("mesh.plan.apply", &json!({"plan": plan["name"]})).unwrap();
+        assert!(r["layout"].as_str().unwrap().contains("10 circle(s) ringed"), "{r}");
+        accept(&mut s, &r, 0.5, 10.0);
+    }
+
     #[test]
     fn a_filleted_box_meshes_structured_round_its_corners() {
         let (_d, mut s) = open(json!([