feat(guilds): declare a second place on the guild page, for core's forum

The mirror of the activity feed, one phase later. Core owns the Team forum —
membership, manual grants and the member/guest split are all core's rules, and a
module reimplementing any of them would be reimplementing a security boundary — but
core publishes no Team page, because it does not own the word "guild". So this
module declares the place and core puts the forum in it.

TWO declarations rather than one, and that is the interesting part. A slot holds one
component and the first fill wins, so folding the forum into `uo.guild.detail`
alongside the feed would hand core the decision about where each of its two
contributions sits on a page this module owns. Separate slots also keep them
independent: with the forum switched off, the feed renders exactly as before.

The registration test now asserts the set of declared slots and that EVERY one of
them is rendered by the page that owns it, rather than naming a single slot twice.
A slot nothing renders is a slot core fills into the void.

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

client/src/routes/public/Guild.jsx

@@ -102,6 +102,11 @@ export default function Guild() {
                 that does not fill it, or when there is nothing to show. The guild
                 is named in OUR terms — core maps its own Team from these two. */}
             <Slot name="uo.guild.detail" externalId={String(id)} moduleId="uo" />
+
+            {/* And the Team forum, in its own place below the feed. Core resolves
+                who may read it — membership and manual grants are core's rules —
+                so this module renders the room and never its door policy. */}
+            <Slot name="uo.guild.forum" externalId={String(id)} moduleId="uo" />
           </>
         )}
       </div>