Function persist_session_id 

pub(crate) fn persist_session_id(
    block_id: &str,
    sid: &str,
    wstore: &Option<Arc<Store>>,
    event_bus: &Option<Arc<EventBus>>,
)

Persist a newly captured session ID to block metadata and broadcast the waveobj:update event so the frontend reflects the change immediately.

This is the “careful” path from persistent.rs and subprocess.rs. ACP was missing this step (it only set inner.session_id in memory) — A5 fixes it by routing ACP through this same function.

No-ops silently when wstore is None (e.g. in unit tests that don’t wire up a store).