feat(ui): FA3 -- mount the social panel shell (Friends/Allegiance/Fellowship/Squelch)

Campaign FA slice FA3: retail's four-tab social panel (LayoutDesc
0x2100006E slot 0x1000018F, RetailPanelCatalog id 12), built on the OP3
OptionsPanelController recipe -- Type-8 tab host, ActivateTabBehavior,
per-page scoped controllers.

- SocialPanelController mounts the tab host and wires the close button;
  F3 (ToggleAllegiancePanel) and F4 (ToggleFellowshipPanel) open the
  panel and switch to their own tab, sharing the same gmPanelUI
  one-active-panel exclusivity every sibling main panel already has.
- The live-DAT tab table CORRECTS the coordinator addendum's x-order
  guess: button 0x1000028C ("Allegiance") pairs with page 0x10000291 and
  is the authored DEFAULT entry, not Friends -- each button's own page
  id and its own P0x57 (matching the F3/F4 ActionMap ids on the
  Allegiance/Fellowship pages specifically) both corroborate the real
  pairing. See SocialPanelController's class doc for the full table.
- SocialFellowshipPageController swaps the two authored empty/full
  frames (0x1000026B/0x10000275) on RuntimeFellowshipState's
  IsInFellowship -- both frames' full containment (name box, create
  button, checkboxes vs. roster list, six buttons) was confirmed by the
  live-mount probe, so a single Visible toggle per frame is the whole
  swap (closes lane-A unknown U6).
- SocialAllegiancePageController hides the monarch/patron blocks and
  blanks their name text to a literal space when
  RuntimeAllegianceState.Snapshot.HasProfile is false, using SCOPED
  FindDescendant lookups (the panel authors 0x10000492 twice, once per
  block).
- SocialFriendsPageController/SocialSquelchPageController bind their
  ListBoxes read-only to RuntimeCommunicationState's existing J4.1
  Friends/Squelch owners (names only), rebuilding on revision change.
  Their action buttons are honest INERT (D1) -- register row AD-79.
- UiTemplateListBox gains Flush() (lane A/D's "Gap found" prerequisite)
  so a poll-and-rebuild list can shrink between refreshes.
- RetailPanelCatalog.SocialPanel = 12, byte-verified from the live slot's
  own P0x10000029; listed in Mounted only (no toolbar button -- lane A
  §6.1: the open path is keyboard-only).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.App/UI/Layout/SocialFellowshipPageController.cs

@@ -0,0 +1,78 @@
+using System;
+using AcDream.Runtime;
+
+namespace AcDream.App.UI.Layout;
+
+/// <summary>
+/// Campaign FA slice FA3: the Fellowship page's empty-state frame swap —
+/// the ONLY behavior this SHELL slice owns for this page (roster rows,
+/// vitals, create dialog, and the option row un-dims are FA4 scope).
+///
+/// <para>
+/// Retail <c>gmFellowshipUI::Update @0x0048F440</c>
+/// (docs/research/2026-08-11-fa-panel-structure.md §4.5):
+/// <c>m_pNotInAFellowshipFrame</c> (<c>0x1000026B</c>) shows and
+/// <c>m_pInAFellowshipFrame</c> (<c>0x10000275</c>) hides when
+/// <c>m_pFellowship == null</c>, and vice-versa. Both are AUTHORED sibling
+/// containers of the fellowship page (<c>0x10000292</c>) — confirmed by the
+/// FA3 live-mount probe: <c>0x1000026B</c> holds the name-entry box, Create
+/// button, and all three visible option checkboxes; <c>0x10000275</c> holds
+/// the roster list and all six member-management buttons. A single
+/// <see cref="UiElement.Visible"/> toggle on each container is therefore the
+/// WHOLE empty-state swap; no per-child hiding is needed (closes lane-A
+/// unknown U6).
+/// </para>
+/// </summary>
+public sealed class SocialFellowshipPageController
+{
+    private readonly UiElement _notInFellowshipFrame;
+    private readonly UiElement _inFellowshipFrame;
+    private readonly Func<RuntimeFellowshipSnapshot> _snapshot;
+
+    private SocialFellowshipPageController(
+        UiElement notInFellowshipFrame,
+        UiElement inFellowshipFrame,
+        Func<RuntimeFellowshipSnapshot> snapshot)
+    {
+        _notInFellowshipFrame = notInFellowshipFrame;
+        _inFellowshipFrame = inFellowshipFrame;
+        _snapshot = snapshot;
+    }
+
+    public static SocialFellowshipPageController? Bind(
+        UiElement pageRoot,
+        Func<RuntimeFellowshipSnapshot> snapshot)
+    {
+        ArgumentNullException.ThrowIfNull(pageRoot);
+        ArgumentNullException.ThrowIfNull(snapshot);
+
+        if (UiElement.FindDescendant(pageRoot, 0x1000026Bu) is not { } notIn
+            || UiElement.FindDescendant(pageRoot, 0x10000275u) is not { } inFellowship)
+        {
+            Console.WriteLine(
+                "[D.2b] SocialFellowshipPageController: empty/full frame pair "
+                + "(0x1000026B/0x10000275) not found — fellowship page will not "
+                + "swap its empty state.");
+            return null;
+        }
+
+        var controller = new SocialFellowshipPageController(notIn, inFellowship, snapshot);
+        controller.Tick();
+        return controller;
+    }
+
+    /// <summary>
+    /// Re-reads the live snapshot and applies retail's frame swap. Cheap (two
+    /// bool writes) — called every frame from
+    /// <see cref="SocialPanelController.Tick"/> rather than gated on
+    /// page-shown/revision, since <see cref="UiElement.Visible"/> has no
+    /// data-driven provider mechanism of its own (unlike <see cref="UiText.LinesProvider"/>,
+    /// which is polled by the render loop already).
+    /// </summary>
+    public void Tick()
+    {
+        bool inFellowship = _snapshot().IsInFellowship;
+        _notInFellowshipFrame.Visible = !inFellowship;
+        _inFellowshipFrame.Visible = inFellowship;
+    }
+}