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.
Convert input parsing to Pint (matching the other calculations), carry
units in the YAML values, and drop unit suffixes from input keys, local
variables, results keys, the Typst sheet, and tests.
