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/concrete-beam.typ

@@ -1,4 +1,5 @@
 #import "assets/sheet.typ": calcline, calcsheet, check
+#import "assets/beam-section.typ": beam-section
 
 #let data = json("results.json")
 #let n = data.values
@@ -37,6 +38,16 @@ Simple-span rectangular beam under uniform gravity load. Numbers come from `calc
   caption: [#n.span ft simply supported beam, #n.bw in × #n.h in rectangular section.],
 )
 
+== Section and Reinforcement
+
+#figure(
+  beam-section(),
+  caption: [
+    Beam cross-section: #n.bw in × #n.h in, No. #n.stirrup_size stirrups at #n.stirrup_spacing in
+    on center, and #n.n_bars No. #n.bar_size longitudinal bars (A_s = #n.As in²).
+  ],
+)
+
 == Loads and Beam Demand
 
 #calcline([$L = #n.span " ft"$], [Simple span])