Implement the variable store with deterministic last-write-wins behavior

[Matographo]

Summary

Create the variable-resolution layer that merges values from multiple CLI sources in argument order.

Why

Variable precedence is central to rendering correctness and must be deterministic before expression evaluation is added.

Deliverables

• Variable store
• Merge pipeline
• Last-write-wins semantics
• Foundation for strict and non-strict resolution

Acceptance Criteria

• Duplicate keys use last write wins
• Merge order follows CLI order
• Scalar and structured values are supported
• Resolution behavior is testable in isolation

Dependencies

• Depends on Create the base project structure for CLI, engine, parser, evaluator, and config #3