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 <noreply@anthropic.com>
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.