Update concrete-beam example, section placement, and README

- Longitudinal bars 4 #8 bottom / 2 #8 top; stirrups #3 @ 8 in.
- Move the section figure after Loads and Beam Demand.
- Add concrete-beam/README.md.
- Regenerate results.json, assets/beam-section.typ, and the PDF.

Note: the example-value tests (test_example_flexure,
test_reinforcement_geometry) are not updated for the new input and
currently fail.

concrete-beam/README.md

@@ -0,0 +1,29 @@
+# Concrete Beam
+
+Simple-span rectangular reinforced concrete beam, checked per ACI 318 for flexure, minimum steel, and concrete shear under a uniform gravity load (1.2D + 1.6L).
+
+## Files
+
+- `input.yaml` — geometry, loads, materials, and reinforcement; units are carried in the values.
+- `calc.py` — computes the demands and capacities, writes `results.json`, and regenerates the cross-section figure.
+- `beam_section.py` — renders the cross-section as native Typst (`assets/beam-section.typ`), drawn to a fixed schematic cover.
+- `test_concrete_beam.py` — pytest checks of the demands, flexure, minimum steel, shear, reinforcement geometry, and the generated figure.
+- `concrete-beam.typ` — report; compiles to `concrete-beam.pdf`.
+
+`As` is derived from the provided bars (`n_bars`, `bar_size`).
+
+## Usage
+
+Edit `input.yaml`, then:
+
+```sh
+python3 calc.py                  # writes results.json and assets/beam-section.typ
+python3 -m pytest                # checks the calculation and the generated figure
+typst compile concrete-beam.typ  # builds concrete-beam.pdf
+```
+
+`calc.py` must be run before compiling so the figure matches the current inputs; the tests fail if `assets/beam-section.typ` is stale.
+
+## Requirements
+
+Python 3 with `pyyaml`, `pint`, `pytest`; Typst.