refactor(arena-tk2): FinishAsync returns RunFinishOutcome with WasFullClear signal

Introduces RunFinishOutcome — a semantic record that wraps the wire
FinishResponseDto with the controller-side WasFullClear signal used to fire
the challenge_full_clear mission event. The wire response isn't affected
(controller unwraps the .Response field before returning to the client);
RetireAsync stays returning FinishResponseDto because retire cannot full-clear.

Full-clear detection compares run.WinCount against ResolveMaxBattleCountAsync
rather than hard-coding 5, so a future TK2 rules-config change tracks
automatically. New test covers the full-clear signal explicitly.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.UnitTests/Services/ArenaTwoPickServiceFinishTests.cs

@@ -123,11 +123,24 @@ public class ArenaTwoPickServiceFinishTests
         var (db, svc, vid) = await SetupWithRunAsync(winCount: 0, lossCount: 5);
         await using var _ = db;
 
-        var dto = await svc.FinishAsync(vid);
-        Assert.That(dto.Rewards.Single(r => r.RewardType == 9).RewardCount, Is.EqualTo(100));
+        var outcome = await svc.FinishAsync(vid);
+        Assert.That(outcome.Response.Rewards.Single(r => r.RewardType == 9).RewardCount, Is.EqualTo(100));
+        Assert.That(outcome.WasFullClear, Is.False, "0W 5L is not a full clear");
         Assert.That(await db.ViewerArenaTwoPickRuns.AnyAsync(), Is.False);
     }
 
+    [Test]
+    public async Task FinishAsync_signals_full_clear_when_run_ended_with_5_wins()
+    {
+        // Full clear = 5W 0L (max battles = 5). Controller consumes this to fire the
+        // challenge_full_clear mission event.
+        var (db, svc, vid) = await SetupWithRunAsync(winCount: 5, lossCount: 0);
+        await using var _ = db;
+
+        var outcome = await svc.FinishAsync(vid);
+        Assert.That(outcome.WasFullClear, Is.True);
+    }
+
     [Test]
     public async Task RecordBattleResultAsync_win_increments_win_and_grants_xp_and_spot_points()
     {