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/UiTemplateListBox.cs

@@ -235,4 +235,18 @@ public sealed class UiTemplateListBox : UiDatElement
         viewport.AddChild(row);
         return row;
     }
+
+    /// <summary>
+    /// Campaign FA slice FA3: removes every row previously added via
+    /// <see cref="AddItemFromTemplateList"/>/<see cref="AddPrebuiltRow"/>, resetting
+    /// <see cref="ContentHeight"/> to 0 — the "Gap found" prerequisite lane A/D
+    /// flagged (docs/research/2026-08-11-fa-panel-structure.md §6.6: "no Flush, no
+    /// selection model, no per-row instance-id"). Needed for a poll-and-rebuild
+    /// binding (Friends/Squelch read-only lists) where the row COUNT can shrink
+    /// between refreshes — <see cref="AddItemFromTemplateList"/> alone only ever
+    /// grows the stack. A no-op, never allocating the viewport, when the box is
+    /// still dormant (no row has ever been added) — mirrors every other dormancy
+    /// guard on this class (see class doc).
+    /// </summary>
+    public void Flush() => _viewport?.ClearContent();
 }