feat(gen): add #import for shared schema files

[NathanFlurry]

• Add root-relative #import directives with recursive schema discovery and shared Rust modules, including transitive imports and imported-type derives.
• Reject cycles, missing files, conflicting or shadowed types, and module-name collisions with file-specific diagnostics.
• Document import syntax and module naming, and cover imports with compiled fixtures and round-trip tests. The Rust workspace test suite passes.