Function verify_cross_channel_signature_in 

pub(super) fn verify_cross_channel_signature_in(
    state: &AppState,
    req: &mut InjectionRequest,
    shared_dir: &Path,
    now_secs: i64,
)

verify_cross_channel_signature with the shared registry dir and clock injected — the testable core; the wrapper above only resolves the real dir. Keeps tests off the process-global AGENTMUX_SHARED_DIR env var.