mesh: the tet objective sees needles; no tet below scaled Jacobian 0.3 on the skill's tet parts

The improvement objective was the smallest dihedral sine, which misses
needles: every dihedral angle fine while a corner is nearly flat, so
the corpus meshes had scaled Jacobians down to 0.01-0.07. The objective
is now the smaller of the dihedral sine and the scaled Jacobian at the
worst corner (the measure analysts check, Knupp 2001), weighted so a
Jacobian of 0.3 (the skill's limit) counts as a 30 degree dihedral
angle (the schedule's target): the Jacobian decides only near that
limit. The existing edge removal, multi-face removal, smoothing and
insertion then remove the needles.

Skill tet10 min SJ: sphere-on-plate 0.092 -> 0.335, filleted-bracket
0.021 -> 0.318 (both pass, nothing marked). Default-size tet10:
thin-ribs 0.240, shallow-groove 0.321, rounded-box 0.311.

The strong-relaxation test relaxes below 0.45: nothing is left below
0.3. Literature note in docs/meshing/literature.md.

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

crates/engine/src/cmd/relax/tests.rs

@@ -275,7 +275,8 @@ fn on_box_edge(p: [f64; 3]) -> bool {
 fn strong_relaxation_of_a_filleted_box_keeps_faces_and_edges() {
     let (_d, mut s, _) = filleted_box();
     let before = s.model.clone();
-    let r = s.execute("mesh.node.relax", &json!({"mode": "strong", "bodies": ["B"], "below": 0.3})).unwrap();
+    // The tet mesher leaves no element below the skill's 0.3, so relax those below 0.45.
+    let r = s.execute("mesh.node.relax", &json!({"mode": "strong", "bodies": ["B"], "below": 0.45})).unwrap();
     let (b, a) = (&r["before"], &r["after"]);
     // The worst tets may not improve: their corners can all be held on the box's edges.
     assert!(a["min_scaled_jacobian"].as_f64().unwrap() >= b["min_scaled_jacobian"].as_f64().unwrap(), "{r}");