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/testkit/src/skill.rs

@@ -52,10 +52,6 @@ const PLAIN_CUP: &str = r#"[
 /// Why a part fails the skill's limits today, read from its numbers and its section picture.
 /// Shown only while the part fails.
 const CAUSES: &[(&str, &str)] = &[
-    (
-        "filleted-bracket",
-        "no structured method takes the whole body (the plan has no cuts for the fillet beside the holes), and tet10 at the plan's size leaves slivers (min SJ 0.04, 1709 marked). The same part partitioned into mapped cells (`filleted-bracket-blocks`) passes: partition (§3.4).",
-    ),
     (
         "loft-transition",
         "the lofted sides are swept one to one, and the elements where the section changes shape are skewed (min SJ 0.19). Relaxing the four marked elements did not raise it: the blocks are wrong, not the nodes (§6.3). A loft has no template yet (`unmapped`).",
@@ -64,7 +60,6 @@ const CAUSES: &[(&str, &str)] = &[
         "sharp-wedge",
         "the wedge's sharp edge is swept as hex, so every element tapers to it (min SJ 0.01, edge ratio 150). A column of wedge6 at the edge (§4) raises the SJ but not the ratio: any element with a corner on a 2.9° edge has sides at least 1/tan 2.9° ≈ 20 times its width there, twice the limit. The edge has to be blunted (defeatured) or the limit waived along it.",
     ),
-    ("sphere-on-plate", "no structured method takes the plate with the sunk sphere, and tet10 leaves slivers (min SJ 0.12, 662 marked)."),
     (
         "analytic-sphere",
         "no structured method takes a sphere (it has no face to sweep from or axis to revolve about), so the plan does not apply and the fallback's sphere template meshes it at the plan's size (2.45 mm, which keeps a sphere's chords to 0.75 % on paper), 1.1 % short of the volume: the template's elements at the cube's eight corners span more of the sphere than the size. A sphere template in the plan (§3) would size its caps; hex20 (`order: 2`) closes it too.",