Implement the recursive-descent parser for expressions and control flow

[Matographo]

Summary

Parse variable output, expressions, conditionals, loops, and nesting into a stable internal representation.

Why

The language core depends on a parser that preserves semantics while staying predictable and efficient.

Acceptance Criteria

• Expression and block parsing works for documented forms
• Parse errors include useful location context
• Parser output fits the evaluator and renderer pipeline

Dependencies

• Depends on Specify the template language grammar #1
• Depends on Implement a simple high-performance lexer #16