demo video: a horizontal cantilever, checked against beam theory

The demo used the corpus I-beam, which is extruded along z and so stood
upright. A new corpus part, cantilever-i-beam, is the same section on the
YZ plane extruded along x, web upright: a sweep along a horizontal axis
for the meshers, and a cantilever that bends downward on screen. The
i-beam part is unchanged (its corpus row and the meshing tests keep it).

The demo clamps it at x = 0 and loads the free end with 1000 N in -z
through a kinematic coupling, and now checks the solution against beam
theory and stops when it is off: tip deflection (bending plus shear)
within 3 %, root bending stress (stress_xx along the top flange,
extrapolated to the root, nodes from the Abaqus deck) within 2 %.

docs/meshing/corpus.md regenerated with the new part.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

tests/corpus/cantilever-i-beam/README.md

@@ -0,0 +1 @@
+The 200 mm I-beam of `i-beam` lying along X with its web upright (Z up): a sweep along a horizontal axis, and the cantilever of the README demo.

tests/corpus/cantilever-i-beam/recipe.json

@@ -0,0 +1,7 @@
+{"corpus": {"categories": ["sweepable", "thin-wall"], "volume_mm3": 61600},
+ "commands": [
+  {"command": "sketch.create", "params": {"plane": "YZ", "name": "Section"}},
+  {"command": "sketch.line", "params": {"points": [[-20, 0], [20, 0], [20, 3], [1, 3], [1, 37], [20, 37], [20, 40], [-20, 40], [-20, 37], [-1, 37], [-1, 3], [-20, 3]], "closed": true}},
+  {"command": "sketch.finish", "params": {}},
+  {"command": "solid.extrude", "params": {"distance": 200, "body_name": "Beam"}}
+ ]}