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/ui-egui/src/samples.rs

@@ -23,6 +23,7 @@ const THUMBNAILS: &[(&str, &[u8])] = &[
     ("cantilever", include_bytes!("../thumbnails/cantilever.png")),
     ("impact", include_bytes!("../thumbnails/impact.png")),
     ("parts", include_bytes!("../thumbnails/parts.png")),
+    ("ibeam", include_bytes!("../thumbnails/ibeam.png")),
 ];
 
 /// The thumbnails' aspect ratio (height over width).