feat(dashboard): add standalone C0R3 page (system tab → its own page)

Carves the dashboard's SYST3M tab content out into a standalone page at
/core.html, reached from the H0M3 hub, with a createTabStrip sub-tab
strip (default = Rebuild Queue): rebuild queue, meta inputs, kept state,
container load. Same minimal-chrome standalone-page pattern as
/logs.html (← home back-link + sub-tab nav).

This is the additive first step: the new page is its own esbuild bundle
that cold-loads /api/state and subscribes to /dashboard/stream for the
same live rebuild_queue_changed / meta_inputs_changed / meta_update_running
/ tombstones_changed events the dashboard uses. The four section
renderers are ported from tabs.js; the dashboard's SYST3M tab is left in
place and untouched, so this PR cannot regress the existing dashboard.
Removing the now-duplicate SYST3M tab + de-duplicating the renderers
(into a shared module) + splitting the shared section CSS out of
dashboard.css is the deliberate follow-up.

core.css imports dashboard.css wholesale (transitional) so the ported
sections render identically; dashboard.css does not import common.css so
nothing double-loads. build.mjs gains core.js / core.css / core.html
entries; no Rust change (hive-c0re serves dist/ via ServeDir, and the
/dashboard/stream + /dashboard/history routes are registered ahead of
the fallback).

Page name "C0R3" is a placeholder pending the operator's pick — trivially
renamed (the /core.html URL + the C0R3 label).

frontend/packages/dashboard/src/core.css

@@ -0,0 +1,36 @@
+/* ─── /core.html — C0R3 page ───────────────────────────────────────
+   Standalone page carved out of the dashboard's old SYST3M tab. Same
+   minimal-chrome pattern as /logs.html (a `← home` back-link + a
+   createTabStrip sub-tab strip; base tab + page-header styling comes
+   from @hive/shared via common.css, linked by core.html).
+
+   The four section renderers (rebuild queue, meta inputs, kept state,
+   container load) are ported verbatim from the dashboard, so they emit
+   the exact same class names. Those section-internal rules live in
+   dashboard.css; rather than duplicate the (scattered) blocks here, we
+   @import dashboard.css wholesale — transitional. dashboard.css does
+   NOT @import common.css (it relies on link order), so this doesn't
+   double-load anything; the dashboard-only chrome rules it carries
+   (`body.dashboard-shell`, `.dashboard-chrome`, `.tabbar`) simply match
+   nothing on /core.html. The follow-up that removes the SYST3M tab can
+   split the shared section rules into their own stylesheet. */
+@import "./dashboard.css";
+
+body.core-shell {
+  margin: 0;
+  padding: 0;
+}
+
+/* Fill the header row next to the ← home back-link, same as the logs
+   sub-tab strip. */
+.core-tabbar {
+  flex: 1;
+}
+
+.core-main {
+  padding: 1.2em 1.5em 2em;
+}
+
+/* createTabStrip toggles the native `hidden` attribute on inactive
+   panes; ensure it wins over any inherited display. */
+.core-pane[hidden] { display: none; }