fix(#2854): slice the store-path hash from the front, not the pname suffix from the back

current_flake_rev canonicalizes to /nix/store/<hash>-<pname>; the
hash right after /nix/store/ is what varies between builds, the
trailing -<pname> is constant. slice(-12) was taking the tail, so
two different builds would very likely render the same truncated
string. slice the hash prefix out instead, with a plain head-slice
fallback for a non-store-path rev (e.g. a bare local dir in dev).

damocles caught this in review on PR #2869.

frontend/packages/dashboard/src/home.js

@@ -48,6 +48,26 @@ async function init() {
     ident.textContent = parts.join(' / ');
     ident.hidden = false;
   }
+
+  // Rev line: absent when the flake ref isn't a local path pin (a bare
+  // `github:` url has no on-disk rev to canonicalize) — hidden rather
+  // than showing a placeholder in that case.
+  //
+  // `current_flake_rev` canonicalizes to a nix store path
+  // (/nix/store/<hash>-<pname>) — the part that actually varies between
+  // builds is the hash right after `/nix/store/`, not the trailing
+  // `-<pname>` (constant across revs). Slice the hash out of the front
+  // rather than truncating from the tail, so two different builds don't
+  // render as the same string. Falls back to a plain head-slice for a
+  // non-store path (e.g. a bare local dir during dev).
+  const rev = $('hive-rev');
+  if (rev && state.hyperhive_rev) {
+    const storeMatch = state.hyperhive_rev.match(/^\/nix\/store\/([^-]+)/);
+    const short = storeMatch ? storeMatch[1].slice(0, 12) : state.hyperhive_rev.slice(0, 12);
+    rev.textContent = `rev ${short}`;
+    rev.title = state.hyperhive_rev;
+    rev.hidden = false;
+  }
 }
 
 init();

frontend/packages/dashboard/src/index.html

@@ -30,6 +30,7 @@
   <header class="home-header">
     <p class="banner-thin" id="hive-identity" hidden></p>
     <pre class="banner">░▒▓█▓▒░  ░▒▓█▓▒░  H Y P E R H I V E  ·  H0M3  ░▒▓█▓▒░  ░▒▓█▓▒░</pre>
+    <p class="banner-thin" id="hive-rev" hidden></p>
   </header>
 
   <main class="home-main">

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,