Implement built-in function-like variables and runtime metadata helpers

[Matographo]

Summary

Add a reserved built-in namespace for values such as time, date, current file metadata, and loop metadata.

Why

Built-ins reduce template boilerplate and allow useful runtime metadata without arbitrary code execution.

Acceptance Criteria

• Built-ins are documented and deterministic where possible
• Built-ins do not collide with user variables
• Built-ins can be used in expressions and control flow

Dependencies

• Depends on Specify the template language grammar #1
• Depends on Implement the recursive-descent parser for expressions and control flow #17