Function fileSchemaIdentifier

The identifier a STEP writer declares in FILE_SCHEMA for a schema family.

ifc-lite names schemas by family ('IFC2X3' | 'IFC4' | 'IFC4X3' | 'IFC5'), and for three of them the family name is also the file identifier. IFC4X3 is the exception (#5351). The attribute layouts ifc-lite writes for IFC4X3 are those of IFC4X3_ADD2 (ISO 16739-1:2024), but IfcOpenShell 0.8.x (and so the buildingSMART Validation Service built on it) resolves the bare IFC4X3 token to a later DEVELOPMENT schema whose layouts differ: IfcTriangulatedIrregularNetwork puts Closed before Normals, IfcMapConversion has 10 attributes instead of 8. Standard-conformant output then fails ifcopenshell.validate purely because of the token.

This is the one place the TypeScript writers get that token from; the Rust twin is rust/export/src/file_schema.rs. Readers are unaffected: every reader in the repo already resolves IFC4X3_ADD2 to the IFC4X3 family.

Only a writer that CHOOSES the token calls this. A re-export that preserves the source file's own FILE_SCHEMA token (round-trip header fidelity) does not: it cannot know which layouts a file declared as bare IFC4X3 carries.