refactor: EvalContext struct to reduce parameter threading

[dean0x]

Problem

The evaluator function signatures thread 5 parameters across the codebase:

• scope: &mut Scope
• call_stack: &HashSet<String>
• total_iterations: &mut u64
• warnings: &mut Vec<String>
• depth: usize

This parameter threading is error-prone and makes function signatures hard to read.

Solution

Bundle these into an EvalContext struct:

pub struct EvalContext {
    scope: &mut Scope,
    call_stack: &HashSet<String>,
    total_iterations: &mut u64,
    warnings: &mut Vec<String>,
    depth: usize,
}

Pass &mut EvalContext to all evaluator functions.

Files

• src/evaluator.rs — Create struct and refactor signature threading

Benefits

• Reduces function parameter count from 5+ to 1
• Clearer intent (all evaluator state grouped logically)
• Easier to add future context fields
• Improves readability of evaluator call sites

Acceptance Criteria

• EvalContext struct created in evaluator module
• All evaluator functions refactored to use &mut EvalContext
• Parameter threading reduced (verifiable through signature change)
• All existing tests pass
• No behavior change to evaluation semantics

Related Tech Debt

This is independent of but complements the HashSet→Vec call_stack refactoring (RUST-2).

Co-Authored-By: Claude noreply@anthropic.com