fix: use SetNode type for Set<string> allocation, fixing native compiler codegen bug

[cs01]

Summary

• Fix native compiler generating numeric Set codegen for Set<string> variables
• Root cause: inline type assertion as { valueType?: string } on SetNode reads field at wrong struct index — native compiler accesses index 0 instead of index 2 for valueType
• Fix: replace with proper as SetNode from ast/types.ts (already imported)
• Un-skip the set-string-operations test — now tests add, has, delete, and size for Set<string>

Test plan

• set-string-operations.ts passes with both JS and native compiler
• verify:quick passes including self-hosting Stage 1