Phase 21.8 foundation for MatI64/IntArrayCore integration
Changes:
- Add `imports: HashMap<String, String>` to BridgeEnv
- Extend MapVars::resolve() to check imports and create static box references
- Add BridgeEnv::with_imports() to initialize with using imports map
- Add parse_json_v0_to_module_with_imports() to json_v0_bridge
- Add program_json_to_mir_json_with_imports() to mir_builder.rs
- Maintain backward compatibility via empty HashMap defaults
Next: Wire using extraction from pipeline and test with MatI64
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Co-Authored-By: Claude <noreply@anthropic.com>