Element quality metrics and the certified validity gate (mesh.quality) (#12)

* mesh: element quality metrics and a certified Jacobian validity check

Bernstein-form Jacobian determinants for tet4/tet10/wedge6/hex8/hex20, certified
by bounds and subdivision (Johnen-Remacle-Geuzaine 2013); Shewchuk 2002 tet
metrics, Knupp 2001 hex/wedge/quad metrics; quality report and the validity
gate meshers run on their output.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* engine: mesh.quality command; model documents its node order

mesh.quality reports certified validity, volume and per-metric statistics with
fixed-range histograms. The model's ElementKind doc states the Abaqus/VTK node
order and that deck code reorders formats like Nastran CHEXA20.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* quality: simplify after review; exact orient3d for tet4; gate on raw mesher arrays

- tet4 validity from the exact orient3d of the input coordinates
- validity_gate_elements takes (label, kind, coordinates) straight from a mesher
- one Jacobian per solid; certify caches piece bounds; product weights precomputed
- model::check_coordinates shared by set_node and quality; ElementKind is Ord
- engine: shared labels() parameter parser

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

---------

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/mod.rs

@@ -4,6 +4,7 @@ mod geometry;
 mod mesh;
 mod model;
 
+use modelcraft_model::MAX_LABEL;
 use serde_json::Value;
 
 use crate::{CommandSpec, EngineError, Result};
@@ -18,6 +19,7 @@ pub static ALL: &[CommandSpec] = &[
     mesh::NODE_REMOVE,
     mesh::ELEMENT_ADD,
     mesh::ELEMENT_REMOVE,
+    mesh::QUALITY,
 ];
 
 /// A required label parameter (1..=u32::MAX; the model checks its own range).
@@ -38,3 +40,16 @@ pub(crate) fn xyz(p: &Value, key: &str) -> Result<[f64; 3]> {
     }
     Ok(out)
 }
+
+/// A required list of labels (each in 1..=MAX_LABEL), at most `max_len` long.
+pub(crate) fn labels(p: &Value, key: &str, max_len: usize) -> Result<Vec<u32>> {
+    let raw = p
+        .get(key)
+        .and_then(Value::as_array)
+        .filter(|a| a.len() <= max_len)
+        .ok_or_else(|| EngineError::Params(format!("`{key}` must be a list of at most {max_len} labels")))?;
+    raw.iter()
+        .map(|v| v.as_u64().and_then(|n| u32::try_from(n).ok()).filter(|n| (1..=MAX_LABEL).contains(n)))
+        .collect::<Option<Vec<u32>>>()
+        .ok_or_else(|| EngineError::Params(format!("`{key}` must be labels in 1..={MAX_LABEL}")))
+}