Implement expression evaluation for nested variables, member access, and indexing

[Matographo]

Summary

Support structured data access such as user.name, items[0], and deeper nested expressions.

Why

Nested access and safe missing propagation are central language guarantees.

Acceptance Criteria

• Nested object access works
• Array indexing works
• Out-of-range and absent lookups evaluate to missing
• Strict mode converts unresolved access into a render failure when required

Dependencies

• Depends on Specify and implement external structured variable input #10
• Depends on Implement the recursive-descent parser for expressions and control flow #17