feat(runtime): FA4 -- fellowship roster enumeration + Social selection source

IRuntimeFellowshipView.GetMembers() gives the fellowship panel a way to
enumerate the whole roster (TryGetMember alone needs the guid first,
which a UI roster build doesn't have yet). Implemented on
RuntimeFellowshipState.FellowshipView as a materialized snapshot under
the same lock every other read there uses.

SelectionChangeSource.Social covers a fellowship-roster row click
(gmFellowshipUI's list-selection arm calls the same
ACCWeenieObject::SetSelectedObject primitive every other selection
origin uses -- lane B docs/research/2026-08-11-fa-panel-structure.md
§6.2/§2.8).

Runtime tests: +4 (RuntimeFellowshipStateTests.GetMembers_*).

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

src/AcDream.Runtime/Gameplay/RuntimeFellowshipState.cs

@@ -406,6 +406,24 @@ public sealed class RuntimeFellowshipState : IDisposable
             }
         }
 
+        /// <summary>
+        /// Campaign FA slice FA4: snapshots the whole roster under the same
+        /// lock every other read here uses. Returns a materialized array
+        /// (not a lazy iterator over live state) so the caller can walk it
+        /// after the lock releases without racing a concurrent mutation.
+        /// </summary>
+        public IEnumerable<RuntimeFellowMemberSnapshot> GetMembers()
+        {
+            lock (owner._gate)
+            {
+                var result = new RuntimeFellowMemberSnapshot[owner._members.Count];
+                int i = 0;
+                foreach (GameEvents.FellowMember raw in owner._members.Values)
+                    result[i++] = ToSnapshot(raw);
+                return result;
+            }
+        }
+
         private static RuntimeFellowMemberSnapshot ToSnapshot(
             GameEvents.FellowMember raw) =>
             new(