Refine beam section figure and derive As from bars

- Derive As from the provided longitudinal bars instead of taking it as
  input; drop the redundant As_bars output and update tests.
- Draw a fixed schematic 1 in cover and pin the bottom bars to the
  stirrup's bottom leg so the figure no longer depends on cover or d.
- Fixed stirrup stroke with a rounded 3 pt corner radius.
- Right-align the figure, constrain its caption to the image width, and
  shorten the caption to "Beam cross-section".
- Document input.yaml fields.

concrete-beam/input.yaml

@@ -1,23 +1,42 @@
+# Project name shown in the Typst report header.
 project: "Deer Creek Shoring"
+# Person or organization shown in the Typst report footer.
 prepared_by: "Conemco Engineering"
 
+# Simple span used for the moment and shear demands.
 span: "16 ft"
+# Tributary width carried by the beam.
 tributary: "6.25 ft"
+# Dead load including superimposed dead (psf).
 D: "55 psf"
+# Live load (psf).
 L: "20 psf"
+# Beam web width b_w.
 bw: "8 in"
+# Overall beam depth h.
 h: "12 in"
+# Effective depth to the tension-steel centroid, used for flexure.
 d: "9.5 in"
+# Concrete compressive strength f'_c.
 fc: "3.0 ksi"
+# Steel yield strength f_y.
 fy: "60 ksi"
-As: "0.62 in**2"
+# Concrete unit weight used for the beam self-weight.
 concrete_density: "150 pcf"
+# Factored load combination applied to the demands.
 load_combination: "1.2D + 1.6L"
 
+# Clear cover from the concrete face to the stirrup.
 cover: "1.5 in"
+# Stirrup bar number (#3).
 stirrup_size: 3
+# Stirrup spacing along the span, on center.
 stirrup_spacing: "6 in"
+# Bottom (tension) longitudinal bar number (#5).
 bar_size: 5
+# Number of bottom longitudinal bars.
 n_bars: 2
+# Top (hanger) longitudinal bar number (#4).
 top_bar_size: 4
+# Number of top hanger bars.
 n_top_bars: 2