M4-05: Recover levels and storey assignment

[DrunkOnJava]

Created from the roadmap audit in TODO.md.

Source: https://github.com/DrunkOnJava/rvt-rs/blob/main/TODO.md#L603
Tracking commit: 1ec8105

Scope

Labels: priority:P0, type:feature, area:elements, area:ifc

• Decode Level names/elevations from real project files.
• Assign elements to storeys via level references.
• Remove fallback-only Level 1 behavior for files with real levels.

Acceptance criteria:

• IFC storeys match project levels.
• Elements are grouped under correct storey.
• Viewer scene tree groups by actual levels.

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• The TODO acceptance criteria for this section are implemented or explicitly superseded by a linked decision.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