dashboard: surface the hive's hyperhive rev on the H0M3 start page

Adds hyperhive_rev to the dashboard's /api/state StateSnapshot,
resolved via the same current_flake_rev helper get_agent_meta's
per-agent hyperhive_rev already uses. home.js renders it next to the
existing hive-identity line, truncated to the last 12 chars with the
full value in title=, hidden when the flake ref isn't a local path pin.

Requested by annika (infra.run) via dmatrix, hyperhive#2854.

hive-c0re/src/dashboard/state_snapshot.rs

@@ -37,6 +37,13 @@ pub(super) struct StateSnapshot {
     /// "apply everything you've buffered".
     seq: u64,
     hostname: String,
+    /// Current rev of the pinned `hyperhive` flake input, resolved the
+    /// same way `get_agent_meta`'s per-agent `hyperhive_rev` is (see
+    /// `auto_update::current_flake_rev`). `None` when the flake ref
+    /// isn't a local path pin (e.g. a bare `github:` url) or the rev
+    /// can't be resolved. Feeds the dashboard start page so an operator
+    /// can tell what build a hive is running without shelling in.
+    hyperhive_rev: Option<String>,
     any_stale: bool,
     containers: Vec<ContainerView>,
     transients: Vec<TransientView>,
@@ -382,6 +389,7 @@ pub(super) async fn api_state(
     axum::Json(StateSnapshot {
         seq,
         hostname,
+        hyperhive_rev: crate::auto_update::current_flake_rev(&state.coord.hyperhive_flake),
         any_stale,
         containers,
         transients,