key design outcomes table, braking-load sub-rows, and chapter-local key ownership

[OmPathania21]

Summary

Report improvements across the Executive Summary and Chapter 3, plus a cleanup that
moves report key handling into the chapter modules that use it.

Changes

• Executive Summary: replaced the hardcoded "… design pass" lines with a
  Component | Controlling Check | Utilization Ratio | Status table, driven by the
  worst utilization found in the girder, cross-bracing, end-diaphragm, and deck
  results. Failing components (UR > 1.0) render in red.
• Ch. 3 braking load: now a section with Vehicles Considered, Braking Load Value, and Eccentricity sub-rows. Class SV follows its own braking opt-in
  rather than the live-load selection.
• Ch. 3 fixes: footpath live load, Vz, and Pz now render the computed values
  instead of raw input keys (they were previously blank); unit superscripts emitted
  as \textsuperscript{2} instead of literal ² so LaTeX compiles.
• Cleanup: removed the KEY_* import block from report_generator.py
  (KEY_DESIGN_MODE, KEY_SPAN, KEY_TL_BRIDGE_TEMP_MIN/MAX,
  KEY_TL_HIGHEST_MAX_TEMP, KEY_TL_LOWEST_MIN_TEMP, KEY_TL_TEMP_RISE/FALL) —
  the temperature block and its IRC 6 Cl. 215.2 fallback now live in ch3_loads(),
  which takes output_dict and reads/writes those keys directly. Also drops the
  unused span_m parameter from ch4_analysis() and the dead span_m /
  is_custom locals.