M5-03: Validate real-file IFC outputs

[DrunkOnJava]

Created from the roadmap audit in TODO.md.

Source: https://github.com/DrunkOnJava/rvt-rs/blob/main/TODO.md#L709
Tracking commit: 1ec8105

Scope

Labels: priority:P0, type:test, area:ifc, area:corpus

• For curated real projects, run:
  • rvt-ifc
  • IfcOpenShell open
  • entity count checks
  • spatial hierarchy checks
  • geometry presence checks
  • material/property checks where supported

Acceptance criteria:

• CI validates at least one real-project IFC fixture in addition to
  synthetic fixtures.
• Failures show which IFC entity class regressed.

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• The TODO acceptance criteria for this section are implemented or explicitly superseded by a linked decision.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