fix(rank-battle): route ai-start through the queue-time MatchContext

Live-smoke bug 2026-06-02: queued Bloodcraft (deck #5), wire showed
classId=2 (Swordcraft) for self_info on the /ai_unlimited_rank_battle/start
response — client rendered the wrong leader.

Two layers of the same bug:

1. MatchContextBuilder.BuildForRankBattleAsync hardcoded deckNo=1 instead
   of taking it from the do_matching request — verified against
   data_dumps/captures/traffic.ndjson L17 where deck_no=5 was on the wire.
   Signature changes to (viewerId, format, deckNo); DoMatchingInternal
   passes req.DeckNo.

2. AiStartInternal rebuilt MatchContext from scratch — but the /ai_*/start
   request body is BaseRequest only, no deck_no on the wire. The fix uses
   the MatchContext the bridge already stored at do_matching resolution time
   (in the Bot PendingBattle), so deck/cosmetic data is consistent end-to-end.
   New IBattleSessionStore.TryFindPendingForViewer(viewerId) finds the
   viewer's pending battle for lookup. The store entry persists across
   ai_start (idempotent reads are fine — the WS handler removes on connect).
   No-pending sentinel: ai_id=-1 surfaces the "no AI assigned" error in the
   client.

Tests: 936 → 939 passing.
- MatchContextBuilderTests.BuildForRankBattle_uses_the_caller_supplied_deck_number
  seeds deck #1 (class 1) and deck #5 (class 6) and asserts the deckNo
  argument picks the right one.
- RankBattleControllerTests.AiStart_self_info_class_matches_queued_deck_number
  is the end-to-end regression: register Bot battle with deck #5, hit
  /ai_unlimited_rank_battle/start, assert self_info.classId == 6.
- RankBattleControllerTests.AiStart_without_pending_battle_returns_neg1_sentinel
  locks the defensive ai_id=-1 path.
- Existing AiStart_* tests bypass do_matching, so adapted to call a new
  RegisterBotBattleAsync helper that mirrors what InProcessPairUp does on
  AI-fallback resolution.

SeedDeckAsync gains an optional classId so test cases can differentiate
decks by class (was always picking Classes.First()).

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

SVSim.EmulatedEntrypoint/Services/IMatchContextBuilder.cs

@@ -18,13 +18,10 @@ public interface IMatchContextBuilder
     Task<MatchContext> BuildForTwoPickAsync(long viewerId);
 
     /// <summary>
-    /// Build a context for a rank-battle viewer + format (rotation / unlimited). Pulls the
-    /// viewer's deck #1 for that format + viewer cosmetics. Throws if the viewer has no
-    /// deck registered for the format.
+    /// Build a context for a rank-battle viewer + format (rotation / unlimited) + the
+    /// caller-selected deck number (from <c>do_matching</c>'s <c>deck_no</c>). Pulls the
+    /// viewer's deck for that format/number + viewer cosmetics. Throws if the viewer has
+    /// no deck at that slot.
     /// </summary>
-    /// <remarks>
-    /// Deck-selection persistence (which deck number is "current" for this format) is a
-    /// separate concern; deck #1 is a placeholder until that lands.
-    /// </remarks>
-    Task<MatchContext> BuildForRankBattleAsync(long viewerId, Format format);
+    Task<MatchContext> BuildForRankBattleAsync(long viewerId, Format format, int deckNo);
 }

SVSim.EmulatedEntrypoint/Services/MatchContextBuilder.cs

@@ -70,17 +70,17 @@ public class MatchContextBuilder : IMatchContextBuilder
             BattleType: 11);
     }
 
-    public async Task<MatchContext> BuildForRankBattleAsync(long viewerId, Format format)
+    public async Task<MatchContext> BuildForRankBattleAsync(long viewerId, Format format, int deckNo)
     {
         var viewer = await _viewers.LoadForMatchContextAsync(viewerId)
             ?? throw new InvalidOperationException($"viewer {viewerId} not found");
 
-        // Per spec, deck-selection persistence (which deck number is "current" for this
-        // format) is a separate concern; #1 is a placeholder until that lands. IDeckRepository
-        // is the right path here — viewer-graph nav refs (DeckCard.Card) don't auto-load
-        // (see project_ef_nav_include_pitfall memory), which would silently ship card_id=0.
-        var deck = await _decks.GetDeck(viewerId, format, deckNo: 1)
-            ?? throw new InvalidOperationException($"viewer {viewerId} has no deck for format {format}");
+        // IDeckRepository is the right path here — viewer-graph nav refs (DeckCard.Card)
+        // don't auto-load (see project_ef_nav_include_pitfall memory), which would
+        // silently ship card_id=0.
+        var deck = await _decks.GetDeck(viewerId, format, deckNo)
+            ?? throw new InvalidOperationException(
+                $"viewer {viewerId} has no deck #{deckNo} for format {format}");
 
         var defaults = _config.Get<DefaultLoadoutConfig>();
         var emblemId = viewer.Info.SelectedEmblem.Id != 0