Function collect_async_functions 

pub fn collect_async_functions(stmts: &[Stmt<'_>]) -> HashSet<Symbol>

Phase 54: Collect function names that are async. Used by LaunchTask codegen to determine if .await is needed.

Two-pass analysis:

1. First pass: Collect directly async functions (have Sleep, LaunchTask, etc.)
2. Second pass: Iterate until fixed point - if function calls an async function, mark it async