fix(battle-node): involuntary-drop survivor gets DisconnectWin, not Win=NoContest

Code-review follow-up to the dispatch unification (0a8a84b).

1. The RunAsync drop cascade synthesized BattleFinish(Win=1), which the client
   renders as RESULT_CODE.NoContest ("battle ended in no contest") instead of a
   win. Add DisconnectWin=201 (already in the client enum, routes to WIN UI) and
   ship it for involuntary opponent drops. Update PvpMidGameDisconnect_FullCascade.

2. Remove BuildBattleFinishNoContest() — dead since the Retire/Kill arm moved to
   RetireWin/RetireLose.

3. Correct the BattleResult docstring: Lose/Win/Consistency are no longer emitted
   by any dispatch arm; they survive only as serialization-test constants.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

SVSim.BattleNode/Sessions/BattleSession.cs

@@ -58,11 +58,13 @@ public sealed class BattleSession
 
             if (Phase != BattleSessionPhase.Terminal)
             {
-                // Involuntary drop (no graceful Retire): synthesize BattleFinish(Win) to survivor.
+                // Involuntary drop (no graceful Retire): synthesize BattleFinish(DisconnectWin)
+                // to survivor. DisconnectWin=201 → client renders "opponent disconnected" →
+                // WIN UI; the legacy Win=1 used here previously rendered "no contest".
                 try
                 {
                     await survivor.PushAsync(
-                        BuildBattleFinish(BattleResult.Win), noStock: true, cancellation)
+                        BuildBattleFinish(BattleResult.DisconnectWin), noStock: true, cancellation)
                         .ConfigureAwait(false);
                 }
                 catch (Exception ex)
@@ -334,17 +336,6 @@ public sealed class BattleSession
         PlaySeq: null,
         Body: new ResultCodeOnlyBody());
 
-    private MsgEnvelope BuildBattleFinishNoContest() => new(
-        NetworkBattleUri.BattleFinish,
-        ViewerId: ScriptedLifecycle.FakeOpponentViewerId,
-        Uuid: WireConstants.ServerUuid,
-        Bid: null,
-        Try: 0,
-        Cat: EmitCategory.Battle,
-        PubSeq: null,
-        PlaySeq: null,
-        Body: new BattleFinishBody(Result: BattleResult.Win));
-
     private MsgEnvelope BuildTurnEndBroadcast() => new(
         NetworkBattleUri.TurnEnd,
         ViewerId: ScriptedLifecycle.FakeOpponentViewerId,