CLASS-13: Stair + Railing decoders + IfcStair / IfcRailing emission

[DrunkOnJava]

Problem

Stairs and railings — load-bearing for architectural BIM —
are not yet decoded or emitted.

Scope

• Stair: run type (straight/winder/spiral) + landing + riser/
  tread dimensions -> faceted brep or extruded composites ->
  IFCSTAIR with PredefinedType.
• Railing: path curve + height + post spacing -> swept solid
  -> IFCRAILING.
• Per-storey routing.

Acceptance criteria

• At least 1 IFCSTAIR entity emitted per corpus file with
  stairs.
• At least 1 IFCRAILING entity emitted per corpus file with
  railings.
• Diagnostics list any complex stair runs (helical, multi-run)
  that fall back to bounding-box geometry.

Related

• Per TODO-BLINDSIDE Phase 4 L5B-32/L5B-33