refactor: Arc<FunctionDef> for closure capture (eliminate quadratic cloning)

[dean0x]

Problem

Every @define deep-clones the full scope chain when capturing variables into closures. This creates quadratic behavior when nesting definitions, as each nested definition clones an increasingly large scope.

Scope

This is a refactoring of the ownership model across multiple modules:

• src/scope.rs — Wrap FunctionDef in Arc for shared ownership
• src/resolver.rs — Update closure capture patterns
• src/evaluator.rs — Adjust function invocation logic

Related Tech Debt

• ARCH-4: CapturedScope struct extraction (subsumed by this)
• PERF-4: Arc for cache hits (related ownership model change)

Benefits

• Eliminates quadratic closure capture cloning
• Enables shared ownership of resolver cache via Arc
• Reduces memory pressure in deeply nested @define chains

Acceptance Criteria

• FunctionDef wrapped in Arc across scope.rs, resolver.rs, evaluator.rs
• CapturedScope struct extracted from scope management
• Cache hits in resolver use Arc
• All existing tests pass
• Benchmark shows reduced allocation in nested @define scenarios

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