feat(match-context): add IMatchContextBuilder TK2 implementation

Assembles MatchContext from ArenaTwoPickRun + viewer cosmetics + config.
Per-mode interface — future modes (rank/free/open-room/...) add one method
each. DI scoped registration. Four tests cover happy path, no-run, incomplete
draft, default-loadout fallback.

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

SVSim.EmulatedEntrypoint/Program.cs

@@ -105,6 +105,7 @@ public class Program
         builder.Services.AddScoped<IArenaTwoPickRunRepository, ArenaTwoPickRunRepository>();
         builder.Services.AddScoped<IArenaTwoPickCardPoolService, ArenaTwoPickCardPoolService>();
         builder.Services.AddScoped<IArenaTwoPickService, ArenaTwoPickService>();
+        builder.Services.AddScoped<IMatchContextBuilder, MatchContextBuilder>();
         builder.Services.AddScoped<IStoryService, StoryService>();
         builder.Services.AddScoped<IDeckListBuilder, DeckListBuilder>();
         builder.Services.AddSingleton<IRandom, SystemRandom>();

SVSim.EmulatedEntrypoint/Services/IMatchContextBuilder.cs

@@ -0,0 +1,18 @@
+using SVSim.BattleNode.Bridge;
+
+namespace SVSim.EmulatedEntrypoint.Services;
+
+/// <summary>
+/// Per-mode assembler for the battle-node <c>MatchContext</c>. Each multiplayer mode that
+/// fronts a <c>do_matching</c> endpoint adds one method here that reads its mode-specific
+/// state (TK2 run, current-deck pointer, open-room set_deck, ...) and produces a
+/// <c>MatchContext</c> for the bridge.
+/// </summary>
+public interface IMatchContextBuilder
+{
+    /// <summary>
+    /// Build a context from the viewer's active TK2 run + viewer cosmetics + config.
+    /// Throws <see cref="ArenaTwoPickException"/> on missing run / incomplete draft.
+    /// </summary>
+    Task<MatchContext> BuildForTwoPickAsync(long viewerId);
+}

SVSim.EmulatedEntrypoint/Services/MatchContextBuilder.cs

@@ -0,0 +1,67 @@
+using System.Text.Json;
+using SVSim.BattleNode.Bridge;
+using SVSim.Database.Models.Config;
+using SVSim.Database.Repositories.Viewer;
+using SVSim.Database.Services;
+
+namespace SVSim.EmulatedEntrypoint.Services;
+
+public class MatchContextBuilder : IMatchContextBuilder
+{
+    private readonly IArenaTwoPickRunRepository _runs;
+    private readonly IViewerRepository _viewers;
+    private readonly IGameConfigService _config;
+
+    public MatchContextBuilder(
+        IArenaTwoPickRunRepository runs,
+        IViewerRepository viewers,
+        IGameConfigService config)
+    {
+        _runs = runs;
+        _viewers = viewers;
+        _config = config;
+    }
+
+    public async Task<MatchContext> BuildForTwoPickAsync(long viewerId)
+    {
+        var run = await _runs.GetByViewerIdAsync(viewerId)
+            ?? throw new ArenaTwoPickException("arena_two_pick_no_active_run");
+
+        var deck = JsonSerializer.Deserialize<List<long>>(run.SelectedCardIdsJson) ?? new();
+        if (deck.Count < 30)
+            throw new ArenaTwoPickException("arena_two_pick_draft_incomplete");
+
+        var viewer = await _viewers.LoadForMatchContextAsync(viewerId)
+            ?? throw new ArenaTwoPickException("arena_two_pick_no_active_run");
+
+        var challenge = _config.Get<ChallengeConfig>();
+        var defaults = _config.Get<DefaultLoadoutConfig>();
+
+        var emblemId = viewer.Info.SelectedEmblem.Id != 0
+            ? viewer.Info.SelectedEmblem.Id.ToString()
+            : defaults.EmblemId.ToString();
+        var degreeId = viewer.Info.SelectedDegree.Id != 0
+            ? viewer.Info.SelectedDegree.Id.ToString()
+            : defaults.DegreeId.ToString();
+        var charaId = run.LeaderSkinId != 0
+            ? run.LeaderSkinId.ToString()
+            : run.ClassId.ToString();
+
+        return new MatchContext(
+            SelfDeckCardIds: deck,
+            ClassId: run.ClassId.ToString(),
+            CharaId: charaId,
+            // Hardcoded v1; see spec §Deferred plumbing.
+            CardMasterName: "card_master_node_10015",
+            CountryCode: viewer.Info.CountryCode ?? string.Empty,
+            UserName: viewer.DisplayName,
+            // TK2-specific cosmetic source; other modes will use the deck row's SleeveId.
+            SleeveId: challenge.TwoPickSleeveId.ToString(),
+            EmblemId: emblemId,
+            DegreeId: degreeId,
+            // Hardcoded v1; needs equipped-MyPageBackground lookup (see spec §Deferred).
+            FieldId: 43,
+            IsOfficial: viewer.Info.IsOfficial ? 1 : 0,
+            BattleType: 11);
+    }
+}