M4-06: Recover materials and compound assemblies

[DrunkOnJava]

Created from the roadmap audit in TODO.md.

Source: https://github.com/DrunkOnJava/rvt-rs/blob/main/TODO.md#L617
Tracking commit: 1ec8105

Scope

Labels: priority:P1, type:feature, area:elements, area:ifc

• Decode Material records.
• Decode wall/floor/roof layer stacks.
• Emit:
  • IfcMaterial
  • IfcMaterialLayerSet
  • IfcMaterialLayerSetUsage
• Include colors/transparency where available.

Acceptance criteria:

• Material counts match known fixtures where available.
• IFC material associations validate with IfcOpenShell.
• Viewer applies material names/colors.

Definition of done

• The TODO acceptance criteria for this section are implemented or explicitly superseded by a linked decision.
• Tests, fixtures, or documentation are added at the level appropriate to the change risk.
• User-facing behavior and limitations remain honest in README/docs/viewer messaging.
• Relevant CI checks pass before the issue is closed.