dashboard: meta flake inputs UI + sequential rebuild loop

new section 'M3T4 1NPUTS' between approvals and message flow:
one row per input in meta/flake.lock (hyperhive first, then
agent-<n> alphabetically). each row shows the input name, the
first 12 chars of the locked sha, a relative timestamp from
locked.lastModified, and the original.url when available.
checkbox per row; submit button is disabled until at least one
box is checked; submitting confirms then POSTs the selected
names to /meta-update.

backend:
- meta::lock_update(inputs: &[String]) — runs 'nix flake update
  <names>' in the meta dir, commits the lock change with a
  combined message ('lock update: hyperhive, agent-coder').
  preserves the existing META_LOCK serialization. existing
  lock_update_for_rebuild / lock_update_hyperhive stay for
  their single-input callers.
- POST /meta-update — comma-separated 'inputs' form field
  (JS joins checkboxes since axum::Form doesn't natively
  decode repeated keys); spawns a background task that runs
  the lock update + per-agent rebuild loop. hyperhive
  selection fans out to all agents; agent-<n> selection only
  rebuilds <n>. each rebuild fires Rebuilt to the manager
  exactly like dashboard / admin-CLI / auto-update.

rebuild loop is sequential — auto_update::run too (was
parallel via tokio::spawn). parallel rebuilds collide on
nix-store's sqlite cache ('sqlite db busy, not using cache')
and the meta META_LOCK contention. nix-daemon serializes the
heavy build steps anyway, so this isn't a throughput loss.

hive-c0re/assets/index.html

@@ -47,6 +47,13 @@
     <p class="meta">loading…</p>
   </div>
 
+  <h2>◆ M3T4 1NPUTS ◆</h2>
+  <div class="divider">══════════════════════════════════════════════════════════════</div>
+  <p class="meta">select inputs to <code>nix flake update</code> in <code>/meta/</code>. selected agents rebuild in sequence after the lock bump; manager learns each outcome via the usual <code>rebuilt</code> system event.</p>
+  <div id="meta-inputs-section">
+    <p class="meta">loading…</p>
+  </div>
+
   <h2>◆ MESS4GE FL0W ◆</h2>
   <div class="divider">══════════════════════════════════════════════════════════════</div>
   <p class="meta">live tail — newest at the top. tap on every <code>send</code> / <code>recv</code> through the broker. compose below: <code>@name</code> picks the recipient (sticky until you @ someone else); <code>tab</code> completes.</p>