Function write_active_binding_file 

pub fn write_active_binding_file(workspace_root: &Path, binding_id: &str)

Publish binding_id as the active binding, stale-safe: the previous pointer is unlinked before the new write, so a failed write leaves no pointer (the check refuses NO_ACTIVE_BINDING, consumers fail open) rather than a previous binding’s pointer. Best-effort engine cache, not a tracked mutation.