Implement parsing for -d <file> multi-variable input files

[Matographo]

Summary

Support reading multiple key=value pairs from a variable-definition file.

Why

Batch variable loading is necessary for repeatable automation and larger render contexts.

Acceptance Criteria

• UTF-8 variable files load correctly
• Blank lines and comment lines are ignored
• Later CLI arguments can override loaded values

Dependencies

• Depends on Implement the variable store with deterministic last-write-wins behavior #6