Implement include processing with preprocessing and cycle detection

[Matographo]

Summary

Allow templates to include local files, preprocess them through the same engine, and fail cleanly on circular includes.

Why

Includes are core to modular template composition and require deterministic resolution and cycle safety.

Acceptance Criteria

• Included files are rendered with the same pipeline
• Nested includes work
• Include cycles fail with a clear error

Dependencies

• Depends on Specify the template language grammar #1
• Depends on Implement the recursive-descent parser for expressions and control flow #17
• Depends on Implement the core renderer for strings and single files #22