Function detect_top_level_cd 

pub fn detect_top_level_cd(command: &str) -> Option<String>

Expose applyCwdCarry equivalent for harnesses that want to mutate logical cwd after a successful cd command. Same contract as the TS applyCwdCarry — returns whether the cwd changed and whether the attempt escaped the workspace. Top-level grammar check for a cd <path> command. Returns the target only for a single, unambiguous cd with no shell operators.