engine: drop-test sample, an aluminium bracket dropped onto a tilted floor

file.sample drop-test: an L-bracket standing on its foot (a SolidCraft script
in sample.rs) swept into hex8, falling 2 m onto a rigid floor tilted 15 deg;
explicit dynamics for 1 ms with gravity and plasticity. Its test checks the
mass against rho V, the impact speed sqrt(2 g h) and kinetic energy, the
energy balance and the momentum the floor takes. docs/examples.md starts
with it, and its picture is docs/media/examples/drop-test.png.

Closes #162

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

docs/examples.md

@@ -0,0 +1,28 @@
+# Examples
+
+Every example is a built-in sample: open it from **File ▸ Samples** (or the start screen), with
+`fieldcraft --sample NAME`, or with the `file.sample` command (`{"name": "drop-test"}`), then
+press **Solve** (`solve.run`). Each opens fully defined — geometry, mesh, materials, boundary
+conditions, loads and steps — in mm, t, s, N and MPa, and opens and solves within half a minute. The geometry is our
+own: SolidCraft scripts run when the sample opens (a recipe from [`tests/corpus`](../tests/corpus)
+or one written in `crates/engine/src/sample.rs`). Each sample has a test in
+`crates/engine/src/sample.rs` that opens it, checks it clean, solves it and holds its result to
+the check given below.
+
+## Drop test (`drop-test`)
+
+![The bracket at 0.1 ms after striking the tilted floor, von Mises stress up to 123 MPa in the bend](media/examples/drop-test.png)
+
+An aluminium L-bracket (60 × 40 mm foot, 50 mm upright, 8 mm thick, R6 fillet) falls 2 m onto a
+rigid floor tilted 15°, so the end of its foot strikes first. Explicit dynamics for 1 ms with
+gravity: 1 911 swept hex8 elements (one-point, stiffness hourglass control), bilinear plasticity
+(yield 240 MPa), the floor a rigid wall. About 8 400 increments, a few seconds.
+
+**Look at:** the frames from the strike (Results ▸ Frame): the compression wave leaves the
+struck corner and the upright's inertia bends the fillet, where von Mises peaks near 120 MPa —
+below yield, so the drop leaves no permanent set. The energies plot shows the kinetic energy
+turning into vibration with a balance under 0.01 %.
+
+**Check:** the mass equals ρ V of the bracket (0.5 %); it meets the floor at √(2 g h) =
+6.26 m/s, with ½ m v² = 1 748 mJ of kinetic energy (1 %); energy is conserved (balance < 1 %);
+by 1 ms the floor has taken about half its downward momentum.