fix: propagate arrow function return types and fix lambda pointer codegen

[cs01]

Summary

• Arrow functions with explicit return type annotations (e.g., (): string => { ... }) now propagate the return type through the codegen pipeline
• Parser extracts return type from arrow function declarations and stores in ArrowFunctionNode.returnType
• inferReturnTypeFromBody now handles binary string concat in return statements
• Closure calls use correct return type instead of hardcoded double — string-returning closures now call with i8* return type
• Return statements properly bitcast lambda function pointers to i8* instead of emitting bare function names

Previously, return (): string => { return "hello"; } would emit ret i8* __lambda_0 (invalid LLVM IR — missing @ prefix and type mismatch). Now emits proper bitcast.

Test plan

• npm run verify:quick passes (tests + self-hosting Stage 1)
• No regressions in existing closure tests