Specify and implement external structured variable input

[Matographo]

Summary

Define how arrays, objects, and nested values are passed in from CLI and variable files, then implement that behavior.

Why

Structured data access is part of the language, but it requires a formal external input format to avoid incompatible implementations.

Acceptance Criteria

• Structured input format is documented
• Arrays and objects can be provided externally
• Nested access works on imported structured data

Dependencies

• Depends on Specify the template language grammar #1
• Depends on Implement the variable store with deterministic last-write-wins behavior #6