engine: the I-beam cantilever sample (ibeam, #143)

The README video's model as a sample: the corpus cantilever-i-beam design opened as geometry,
swept all-hex, clamped at F2:start, 1 kN on F2:top through a kinematic coupling; static step
Load. It has a Start-card thumbnail and a gallery capture; a test checks its tip deflection
against beam theory with shear.

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

crates/engine/src/cmd/forms.rs

@@ -1543,6 +1543,7 @@ pub const SAMPLE: &[Field] = &[choice(
         ("cantilever", "Cantilever"),
         ("impact", "Impact"),
         ("parts", "Parts"),
+        ("ibeam", "I-beam cantilever"),
         ("drop-test", "Drop test"),
         ("rubber-seal", "Rubber seal"),
         ("flow-past", "Flow past a bracket"),