Function send_check_source 

pub fn send_check_source(
    source: &str,
) -> Result<Vec<SendDiagnostic>, ParseError>

Phase 4 (work/FINISH_INTERPRETER.md): run the Send/escape analysis over a program.

Parses source and returns the Send/escape diagnostics (empty = the program respects the message-passing + CRDT discipline). Pure analysis — no codegen.