feat: add member claims to the ClaimLink model

Each ClaimLinkMember stores an owner player uid plus that player's
local claim index (resolved fresh at use, not trusted as stable) so
a claim link can reference existing vanilla claims without copying
their data. Buffer claim is intentionally not referenced here; it
resolves on demand via group ownership.

ClaimLink/ClaimLink.cs

@@ -1,10 +1,24 @@
+using System.Collections.Generic;
 using ProtoBuf;
 
 namespace ClaimLink;
 
+[ProtoContract]
+public class ClaimLinkMember
+{
+    [ProtoMember(1)]
+    public string OwnerPlayerUid = "";
+
+    [ProtoMember(2)]
+    public int LocalClaimIndex;
+}
+
 [ProtoContract]
 public class ClaimLink
 {
     [ProtoMember(1)]
     public int GroupId;
+
+    [ProtoMember(2)]
+    public List<ClaimLinkMember> Members = new();
 }