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.EmulatedEntrypoint/Controllers/ArenaTwoPickController.cs

@@ -49,7 +49,11 @@ public class ArenaTwoPickController : SVSimController
     public async Task<IActionResult> Finish([FromBody] FinishRequest _)
     {
         if (!TryGetViewerId(out var vid)) return Unauthorized();
-        return await GuardAsync(() => _svc.FinishAsync(vid));
+        return await GuardAsync(async () =>
+        {
+            var outcome = await _svc.FinishAsync(vid);
+            return outcome.Response;
+        });
     }
 
     private async Task<IActionResult> GuardAsync<T>(Func<Task<T>> action)