Adds import "path/to/file.flatdata"; support to the schema language.

[VeaaC]

Imports are resolved relative to the importing file. Diamond and cyclic imports are handled (deduplication by absolute path, depth-first traversal with visited set).

Code generation uses separate compilation for C++ and Rust: only types from the root file are emitted, with #include directives (C++) or pub use super::.. re-exports (Rust) referencing sibling generated modules. Python/Dot/Flatdata generators remain monolithic — all imported types are inlined.

Generator CLI gains --depfile flag producing Makefile-style dependency output. The CMake macro uses DEPFILE on 3.20+; older versions fall back to depending only on the root schema file.

Changes:

• Grammar extended with import_statement rule
• New importer.py: resolve_imports(path) returns ordered resolved files + import metadata
• builder.py: build_ast_from_file(path) merges multi-file ASTs, tags nodes with source_file and is_local
• SyntaxTree extended with imports, source_file_map, is_local_node()
• Engine.from_file(path) class method for file-based compilation
• BaseGenerator: filter_nodes() / get_import_directives() hooks
• CppGenerator: filters to local nodes, emits #include for imports
• RustGenerator: _import_reexports_for_namespace() computes super:: depth based on namespace nesting + directory traversal depth
• Writer/Inspector updated to use Engine.from_file()
• Removed dead definition parameter from Structure/Enumeration/Constant/Archive.create()