engine: bracket-hex sample, the partitioned bracket meshed all-hex and solved

file.sample bracket-hex: the corpus bracket split into 30 SolidCraft cells,
meshed by mesh.hex.blocks as one conforming hex20 mesh of 3 mm, clamped at
the upright and loaded at the foot. The sample's script finds the clamp and
load faces in every cell that has them. Its test matches the tip deflection
of a fine tet10 solution within 1 % and checks the clamp carries the load.

Closes #166

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/cmd/forms.rs

@@ -808,8 +808,22 @@ pub const FILE_SAVE_AS: &[Field] = &[path("path", "File", true).req()];
 pub const RECOVERY_LIST: &[Field] = &[path("dir", "Folder", false).hint("the recovery folder")];
 pub const RECOVER: &[Field] = &[text("id", "Autosave").req(), path("dir", "Folder", false).hint("the recovery folder")];
 pub const RECOVERY_DISCARD: &[Field] = &[text("id", "Autosave"), flag("all", "All"), path("dir", "Folder", false).hint("the recovery folder")];
-pub const SAMPLE: &[Field] =
-    &[choice("name", "Sample", &[("cantilever", "Cantilever"), ("impact", "Impact"), ("parts", "Parts"), ("drop-test", "Drop test"), ("rubber-seal", "Rubber seal"), ("flow-past", "Flow past a bracket"), ("heated-bracket", "Heated bracket")]).req().def("\"cantilever\"")];
+pub const SAMPLE: &[Field] = &[choice(
+    "name",
+    "Sample",
+    &[
+        ("cantilever", "Cantilever"),
+        ("impact", "Impact"),
+        ("parts", "Parts"),
+        ("drop-test", "Drop test"),
+        ("rubber-seal", "Rubber seal"),
+        ("flow-past", "Flow past a bracket"),
+        ("heated-bracket", "Heated bracket"),
+        ("bracket-hex", "Hex-meshed bracket"),
+    ],
+)
+.req()
+.def("\"cantilever\"")];
 pub const COUNT: &[Field] = &[int("count", "Count", 1, 1_000_000).def("1")];
 
 pub const DECK: &[Field] = &[