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/coordinator.rs

@@ -72,6 +72,14 @@ pub struct Coordinator {
     /// snapshot.
     dashboard_events: broadcast::Sender<DashboardEvent>,
     event_seq: AtomicU64,
+    /// Count of dashboard-triggered `meta-update` runs currently in
+    /// flight. `post_meta_update` returns 200 immediately and does the
+    /// multi-minute `nix flake update` + agent-rebuild ripple in a
+    /// background task, so without this the META INPUTS panel showed no
+    /// sign anything was happening (issue #259). Held via
+    /// `MetaUpdateGuard`; a count > 0 surfaces on `/api/state` as
+    /// `meta_update_running` and via the `MetaUpdateRunning` event.
+    meta_updates_active: AtomicU64,
     /// Last container snapshot seen by `rescan_containers_and_emit`,
     /// keyed by `ContainerView.name`. The rescan diffs a fresh
     /// `container_view::build_all` against this map and emits one
@@ -109,6 +117,33 @@ impl Drop for TransientGuard {
     }
 }
 
+/// RAII guard for the `meta-update` in-progress flag, held for the
+/// duration of a `run_meta_update` background task. Created by
+/// `Coordinator::meta_update_guard`. Drop decrements the active-run
+/// count; the count crossing back to 0 emits
+/// `MetaUpdateRunning { running: false }`, so a concurrent pair of
+/// updates only flips the dashboard flag once.
+pub struct MetaUpdateGuard {
+    coord: Arc<Coordinator>,
+}
+
+impl Drop for MetaUpdateGuard {
+    fn drop(&mut self) {
+        if self
+            .coord
+            .meta_updates_active
+            .fetch_sub(1, Ordering::SeqCst)
+            == 1
+        {
+            self.coord
+                .emit_dashboard_event(DashboardEvent::MetaUpdateRunning {
+                    seq: self.coord.next_seq(),
+                    running: false,
+                });
+        }
+    }
+}
+
 #[derive(Debug, Clone, Copy)]
 pub enum TransientKind {
     /// `lifecycle::spawn` is running (nixos-container create + update + start).
@@ -165,6 +200,7 @@ impl Coordinator {
             transient: Mutex::new(HashMap::new()),
             dashboard_events,
             event_seq: AtomicU64::new(0),
+            meta_updates_active: AtomicU64::new(0),
             last_containers: tokio::sync::Mutex::new(HashMap::new()),
             shutdown_tx,
         })
@@ -218,6 +254,31 @@ impl Coordinator {
         let _ = self.dashboard_events.send(event);
     }
 
+    /// Mark a `meta-update` as in flight and return an RAII guard that
+    /// clears it on drop (including drop-via-panic). The first
+    /// concurrent run emits `MetaUpdateRunning { running: true }`; the
+    /// last one to finish emits `running: false`. The dashboard's META
+    /// INPUTS panel reads the flag to show a disabled "updating…"
+    /// state while the lock bump + rebuild ripple runs (issue #259).
+    pub fn meta_update_guard(self: &Arc<Self>) -> MetaUpdateGuard {
+        if self.meta_updates_active.fetch_add(1, Ordering::SeqCst) == 0 {
+            self.emit_dashboard_event(DashboardEvent::MetaUpdateRunning {
+                seq: self.next_seq(),
+                running: true,
+            });
+        }
+        MetaUpdateGuard {
+            coord: Arc::clone(self),
+        }
+    }
+
+    /// True while at least one dashboard-triggered `meta-update` is
+    /// running. Surfaced on `/api/state` as `meta_update_running` so a
+    /// client that cold-loads mid-update sees the in-progress state.
+    pub fn meta_update_in_progress(&self) -> bool {
+        self.meta_updates_active.load(Ordering::SeqCst) > 0
+    }
+
     /// Emit `ApprovalAdded` immediately after the row is inserted in
     /// sqlite. Caller passes the diff text it already computed (or
     /// `None` for spawn approvals which carry no diff).