M5-05: Add comparison tooling against Revit IFC exports

[DrunkOnJava]

Created from the roadmap audit in TODO.md.

Source: https://github.com/DrunkOnJava/rvt-rs/blob/main/TODO.md#L746
Tracking commit: 1ec8105

Scope

Labels: priority:P2, type:feature, area:ifc, area:corpus

• Build rvt-ifc-compare or add rvt-ifc --compare.
• Compare rvt-rs output against a reference IFC:
  • entity counts
  • storeys
  • bounding boxes
  • object names/types
  • material counts
  • property keys

Acceptance criteria:

• Tool emits JSON and human summary.
• Known divergences are documented and linked to issues.

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• The TODO acceptance criteria for this section are implemented or explicitly superseded by a linked decision.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.