Implement the core renderer for strings and single files

[Matographo]

Summary

Build the main rendering pipeline with variable substitution, expression evaluation, control flow, and output encoding.

Why

This is the central execution path that turns parsed templates and context into final output.

Acceptance Criteria

• String and single-file rendering work for core language features
• Rendering respects active rules and settings
• Render failures are clear and actionable

Dependencies

• Depends on Implement output routing for stdout, output file, and output directory #5
• Depends on Implement the recursive-descent parser for expressions and control flow #17
• Depends on Implement expression evaluation for nested variables, member access, and indexing #18
• Depends on Implement inline and block rendering semantics #19