dashboard: show meta-update progress in the META INPUTS panel

post_meta_update returns 200 immediately and runs the nix flake
update + agent-rebuild ripple in a background task, so the META
INPUTS panel looked idle for the whole multi-minute window (#259).

Track in-flight runs with a Coordinator atomic counter, exposed via
an RAII MetaUpdateGuard held across run_meta_update. Surface it as
the meta_update_running snapshot field plus a MetaUpdateRunning SSE
event (flipped only when the count crosses 0, so concurrent runs
flip the flag once). The panel shows a pulsing in-progress banner
and disables the update button while a run is active.

hive-c0re/src/dashboard_events.rs

@@ -194,4 +194,14 @@ pub enum DashboardEvent {
         seq: u64,
         inputs: Vec<MetaInputView>,
     },
+    /// A dashboard-triggered `meta-update` started (`running: true`) or
+    /// finished (`running: false`). `post_meta_update` returns 200
+    /// immediately and runs the `nix flake update` + agent-rebuild
+    /// ripple in a background task — this event lets the META INPUTS
+    /// panel show a disabled "updating…" state for that whole window
+    /// instead of looking idle (issue #259). Emitted by
+    /// `Coordinator::meta_update_guard` / `MetaUpdateGuard::drop` only
+    /// when the active-run count crosses 0, so concurrent updates flip
+    /// the flag exactly once.
+    MetaUpdateRunning { seq: u64, running: bool },
 }