Function compile_shacl_mapping 

pub fn compile_shacl_mapping(path: &Path) -> Result<MappingProfile, String>

Boot Phase: parse a .shacl.ttl mapping file and compile it into a MappingProfile for use in the zero-allocation stream phase.

Expected predicates in the Turtle file:

• sh:targetClass — the RDF class of each row (used as subject class hint)
• sh:property [ sh:path <URI> ; sh:datatype xsd:T ; qext:sourceColumn "hdr" ]
• qext:sourceJsonKey "key" may substitute qext:sourceColumn for JSON inputs