CLASS-15: CurtainWall + CurtainGrid + CurtainMullion + CurtainPanel decoders

[DrunkOnJava]

Problem

Curtain walls are common in commercial Revit projects but the
multi-component structure (CurtainWall + CurtainGrid +
CurtainMullion + CurtainPanel) is not decoded.

Scope

• Decode CurtainWall as a Wall subtype with grid metadata.
• Decode CurtainGrid lines + mullions.
• Decode CurtainPanel as a Panel filling each grid cell.
• Emit IFCCURTAINWALL aggregation with IFCMEMBER mullions and
  IFCPLATE panels.

Acceptance criteria

• At least 1 IFCCURTAINWALL aggregation emitted per corpus
  fixture with curtain walls.
• Grid + mullion + panel hierarchy preserved.

Related

• Per TODO-BLINDSIDE Phase 4 L5B-34
• Lower priority — not in MVP architectural-core profile