Document the current Rust-to-IFC entity mapping

[jonathanmcmichael]

Background

Rustit is IFC-based, but its current semantic mapping is spread across the Rust types. A concise map will help BIM practitioners review the model without reading every crate.

Suggested scope

Create docs/ifc-mapping.md and link it from the README. Start with this implemented semantic map:

Rustit type	IFC concept
Project	IfcProject
Level	IfcBuildingStorey
Wall	IfcWall
Activity	IfcTask
ActivityRelationship	IfcRelSequence
ElementActivityLink	IfcRelAssignsToProcess
ClassificationReference	IfcClassificationReference association

Acceptance criteria

• distinguish implemented in-memory semantics from future IFC serialization
• note the IFC 4.3 ADD2 target and stable UUID-to-IfcGlobalId boundary
• explain that MasterFormat and UniFormat references retain system and edition metadata, while Rustit does not redistribute licensed tables
• link each Rustit type to its source file
• make no unsupported certification or compliance claims
• Markdown links and formatting are valid