refactor(modules): move core's UO page content behind the two slots

Core declares site.footer.status and admin.users.detail in main.jsx and fills
both itself, under owner id `core` -- the client twin of registries.registerCore()
and the same trick useShardFlags already uses. The rendered page is unchanged;
what changes is that the content now arrives the way a module's will.

The footer's Shard Status link becomes ShardStatusLink.jsx, and UserDetail's six
UO sections become UserShardSections.jsx. Both are files rather than inline
markup so that the client half of phase 3 deletes a registration and a file
instead of editing a core page under extraction pressure -- which is also what
proves the mechanism before anything depends on it.

The user-detail slot is handed userId and not scope. api.admin.userShard is a UO
binding that leaves core with the client half, so a slot passing it would hand a
module something core is about to delete; an extension builds its own client for
the routes it registered at the other end. Core's own fill now does exactly what
the module will.

Verified in a browser against a real chunk (MODULE_API.md 7.7): a throwaway
module fills both slots and renders its own label and target in the footer with
core's linkStyle, and receives userId on the admin page; a deliberate render
failure is contained to that one spot with the slot named in the console; core's
own fills leave the pages byte-identical to before; and with no module installed
both slots render nothing. Zero CSP reports throughout.

Co-Authored-By: Claude <noreply@anthropic.com>

client/src/components/ShardStatusLink.jsx

@@ -0,0 +1,24 @@
+// ── Core's fill for the `site.footer.status` extension slot ────────────────
+//
+// Phase 3, slice 2 of docs/website/MODULE_SYSTEM.md §2.7.1; the contract is
+// MODULE_API.md §3.7.
+//
+// This is the whole of what used to be four lines inline in SiteFooter.jsx, and
+// it is a file now for one reason: `/site/shard` is a UO page, so the link goes
+// when the client half goes, and core should be deleting a registration rather
+// than editing its footer under extraction pressure.
+//
+// Note what core kept and what it handed over. Core owns the position in the row
+// and the separator around it, and passes `linkStyle` so the row stays visually
+// one row. The label, the destination, and the decision to render at all are
+// this file's — which is exactly the division a module inherits.
+
+import { Link } from 'react-router-dom'
+
+export default function ShardStatusLink({ linkStyle }) {
+  return (
+    <Link to="/site/shard" style={linkStyle}>
+      Shard Status
+    </Link>
+  )
+}

client/src/components/SiteFooter.jsx

@@ -1,5 +1,14 @@
 import { Link } from 'react-router-dom'
 import { useSite } from '../contexts/SiteContext.jsx'
+import Slot from '../modules/Slot.jsx'
+
+const FOOTER_SLOT = 'site.footer.status'
+
+// Handed to the extension rather than left for it to guess. A module rendering
+// its own link in this row should look like the row, and the alternative is
+// every module restating core's colours and then drifting from them the next
+// time this footer is themed.
+const LINK_STYLE = { color: 'var(--accent)', textDecoration: 'none' }
 
 export default function SiteFooter() {
   const { contactEmail, siteTitle } = useSite()
@@ -36,10 +45,14 @@ export default function SiteFooter() {
             <a href={`mailto:${contactEmail}`} style={{ color: 'var(--accent)', textDecoration: 'none' }}>
               {contactEmail}
             </a>
-            &nbsp;·&nbsp;
-            <Link to="/site/shard" style={{ color: 'var(--accent)', textDecoration: 'none' }}>
-              Shard Status
-            </Link>
+            {/* A module's spot in the footer, and core supplies only the
+                position and the styling: the label, the target and whether
+                anything renders at all are the module's (MODULE_API.md §3.7).
+                The separator goes through `wrap` rather than sitting beside the
+                slot, so it shares the extension's fate — no module installed and
+                a module whose link throws both render nothing here, rather than
+                the second leaving a stray middot behind. */}
+            <Slot name={FOOTER_SLOT} linkStyle={LINK_STYLE} wrap={(link) => <>&nbsp;·&nbsp;{link}</>} />
             &nbsp;·&nbsp;
             <Link to="/admin/login" style={{ color: '#5d6b7d', textDecoration: 'none' }}>
               Admin