Add generated beam cross-section figure to concrete-beam

Add reinforcement inputs (cover, stirrup size/spacing, longitudinal and
top bar size/count) and emit the derived bar geometry from calc.py.
beam_section.py renders a native Typst drawing of the section to
assets/beam-section.typ, which concrete-beam.typ imports as a figure.
Effective depth d remains authoritative for the tension-steel position.

concrete-beam/test_concrete_beam.py

@@ -1,5 +1,6 @@
 from pathlib import Path
 import importlib.util
+import math
 
 import pytest
 
@@ -13,6 +14,14 @@ spec.loader.exec_module(calc_module)
 compute = calc_module.compute
 
 
+def _load(name: str, filename: str):
+    spec = importlib.util.spec_from_file_location(name, HERE / filename)
+    assert spec is not None and spec.loader is not None
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
 @pytest.fixture
 def result():
     import yaml
@@ -44,3 +53,35 @@ def test_example_min_steel_and_shear(result):
     assert v["phiVc"] == pytest.approx(6.244016, rel=1e-4)
     assert result["checks"]["minimum_steel"]["ok"] is True
     assert result["checks"]["shear"]["ok"] is True
+
+
+def test_reinforcement_geometry(result):
+    v = result["values"]
+    assert v["cover"] == pytest.approx(1.5)
+    assert v["stirrup_size"] == 3
+    assert v["stirrup_db"] == pytest.approx(0.375)
+    assert v["stirrup_spacing"] == pytest.approx(6.0)
+    assert v["bar_size"] == 5
+    assert v["n_bars"] == 2
+    assert v["bar_db"] == pytest.approx(0.625)
+    assert v["top_bar_size"] == 4
+    assert v["n_top_bars"] == 2
+    assert v["top_bar_db"] == pytest.approx(0.5)
+    assert v["As_bars"] == pytest.approx(2 * math.pi * 0.625**2 / 4.0, rel=1e-6)
+
+
+def test_rejects_depth_without_room_for_bars():
+    import yaml
+
+    with (HERE / "input.yaml").open(encoding="utf-8") as handle:
+        inp = yaml.safe_load(handle)
+    inp["d"] = "11.9 in"
+    with pytest.raises(ValueError):
+        compute(inp)
+
+
+def test_section_fragment_is_current(result):
+    module = _load("beam_section", "beam_section.py")
+    expected = module.render(result["values"])
+    actual = (HERE / "assets" / "beam-section.typ").read_text(encoding="utf-8")
+    assert actual == expected