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/results.json

@@ -27,7 +27,18 @@
     "phiMn": 23.961176,
     "As_min": 0.253333,
     "Vc": 8.325383,
-    "phiVc": 6.244037
+    "phiVc": 6.244037,
+    "cover": 1.5,
+    "stirrup_size": 3,
+    "stirrup_spacing": 6.0,
+    "stirrup_db": 0.375,
+    "bar_size": 5,
+    "n_bars": 2,
+    "bar_db": 0.625,
+    "top_bar_size": 4,
+    "n_top_bars": 2,
+    "top_bar_db": 0.5,
+    "As_bars": 0.613592
   },
   "checks": {
     "flexure": {