feat: implement arena-based memory allocation

[YoEight]

Introduce arena-based memory allocation with string interning

Summary

• Replaces heap-allocated AST nodes with arena-based allocation, using lightweight reference handles (ExprRef, StrRef, VecRef, RecRef, TypeRef) for O(1) lookups and reduced memory overhead
• Introduces StringArena for string interning, deduplicating repeated strings (e.g., field names, identifiers) across queries
• Adds TypeArena with freeze/free semantics, allowing the session to reclaim memory between query analyses while preserving baseline type definitions.