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.UnitTests/Services/MatchContextBuilderTests.cs

@@ -0,0 +1,156 @@
+using System.Text.Json;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.DependencyInjection;
+using SVSim.BattleNode.Bridge;
+using SVSim.Database;
+using SVSim.Database.Models;
+using SVSim.EmulatedEntrypoint.Services;
+using SVSim.UnitTests.Infrastructure;
+
+namespace SVSim.UnitTests.Services;
+
+[TestFixture]
+public class MatchContextBuilderTests
+{
+    [Test]
+    public async Task BuildForTwoPick_returns_context_from_run_state()
+    {
+        await using var factory = new SVSimTestFactory();
+        var vid = await factory.SeedViewerAsync();
+        var deck = Enumerable.Range(1, 30).Select(i => 100_011_000L + i).ToList();
+        int emblemId, degreeId;
+
+        using (var seedScope = factory.Services.CreateScope())
+        {
+            var db = seedScope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+            var emblem = await db.Emblems.FirstAsync();
+            var degree = await db.Degrees.FirstAsync();
+            emblemId = emblem.Id;
+            degreeId = degree.Id;
+
+            var viewer = await db.Viewers.FindAsync(vid);
+            viewer!.DisplayName = "Drafter";
+            viewer.Info.CountryCode = "KOR";
+            viewer.Info.IsOfficial = false;
+            viewer.Info.SelectedEmblem = emblem;
+            viewer.Info.SelectedDegree = degree;
+            db.ViewerArenaTwoPickRuns.Add(new ViewerArenaTwoPickRun
+            {
+                ViewerId = vid,
+                EntryId = 1,
+                ClassId = 5,
+                LeaderSkinId = 5_000_001L,
+                SelectedCardIdsJson = JsonSerializer.Serialize(deck),
+                IsSelectCompleted = true,
+                MaxBattleCount = 5,
+                CandidateClassIdsJson = "[1,2,3]",
+                PendingPickSetsJson = "[]",
+                ResultListJson = "[]",
+                NextCandidateId = 1,
+            });
+            await db.SaveChangesAsync();
+        }
+
+        using var scope = factory.Services.CreateScope();
+        var builder = scope.ServiceProvider.GetRequiredService<IMatchContextBuilder>();
+        var ctx = await builder.BuildForTwoPickAsync(vid);
+
+        Assert.That(ctx.SelfDeckCardIds, Is.EqualTo(deck));
+        Assert.That(ctx.ClassId, Is.EqualTo("5"));
+        Assert.That(ctx.CharaId, Is.EqualTo("5000001"));      // LeaderSkinId set
+        Assert.That(ctx.CountryCode, Is.EqualTo("KOR"));
+        Assert.That(ctx.UserName, Is.EqualTo("Drafter"));
+        Assert.That(ctx.EmblemId, Is.EqualTo(emblemId.ToString()));
+        Assert.That(ctx.DegreeId, Is.EqualTo(degreeId.ToString()));
+        Assert.That(ctx.IsOfficial, Is.EqualTo(0));
+        Assert.That(ctx.BattleType, Is.EqualTo(11));
+        // Hardcoded v1 fixtures (see spec §Deferred plumbing)
+        Assert.That(ctx.CardMasterName, Is.EqualTo("card_master_node_10015"));
+        Assert.That(ctx.FieldId, Is.EqualTo(43));
+        // Sleeve comes from ChallengeConfig.TwoPickSleeveId — ShippedDefaults is 0.
+        Assert.That(ctx.SleeveId, Is.EqualTo("0"));
+    }
+
+    [Test]
+    public async Task BuildForTwoPick_throws_when_no_run()
+    {
+        await using var factory = new SVSimTestFactory();
+        var vid = await factory.SeedViewerAsync();
+
+        using var scope = factory.Services.CreateScope();
+        var builder = scope.ServiceProvider.GetRequiredService<IMatchContextBuilder>();
+
+        var ex = Assert.ThrowsAsync<ArenaTwoPickException>(() => builder.BuildForTwoPickAsync(vid));
+        Assert.That(ex!.ErrorCode, Is.EqualTo("arena_two_pick_no_active_run"));
+    }
+
+    [Test]
+    public async Task BuildForTwoPick_throws_when_draft_incomplete()
+    {
+        await using var factory = new SVSimTestFactory();
+        var vid = await factory.SeedViewerAsync();
+
+        using (var seedScope = factory.Services.CreateScope())
+        {
+            var db = seedScope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+            db.ViewerArenaTwoPickRuns.Add(new ViewerArenaTwoPickRun
+            {
+                ViewerId = vid,
+                EntryId = 1,
+                ClassId = 1,
+                SelectedCardIdsJson = JsonSerializer.Serialize(new long[] { 100_011_001L, 100_011_002L }),
+                IsSelectCompleted = false,
+                CandidateClassIdsJson = "[1,2,3]",
+                PendingPickSetsJson = "[]",
+                ResultListJson = "[]",
+                MaxBattleCount = 5,
+                NextCandidateId = 1,
+            });
+            await db.SaveChangesAsync();
+        }
+
+        using var scope = factory.Services.CreateScope();
+        var builder = scope.ServiceProvider.GetRequiredService<IMatchContextBuilder>();
+
+        var ex = Assert.ThrowsAsync<ArenaTwoPickException>(() => builder.BuildForTwoPickAsync(vid));
+        Assert.That(ex!.ErrorCode, Is.EqualTo("arena_two_pick_draft_incomplete"));
+    }
+
+    [Test]
+    public async Task BuildForTwoPick_falls_back_to_default_loadout_when_unequipped()
+    {
+        await using var factory = new SVSimTestFactory();
+        var vid = await factory.SeedViewerAsync();
+        var deck = Enumerable.Range(1, 30).Select(i => 100_011_000L + i).ToList();
+
+        using (var seedScope = factory.Services.CreateScope())
+        {
+            var db = seedScope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+            // No SelectedEmblem / SelectedDegree set → default (Id=0) nav rows.
+            db.ViewerArenaTwoPickRuns.Add(new ViewerArenaTwoPickRun
+            {
+                ViewerId = vid,
+                EntryId = 1,
+                ClassId = 1,
+                LeaderSkinId = 0,               // No skin chosen → CharaId == ClassId
+                SelectedCardIdsJson = JsonSerializer.Serialize(deck),
+                IsSelectCompleted = true,
+                CandidateClassIdsJson = "[1,2,3]",
+                PendingPickSetsJson = "[]",
+                ResultListJson = "[]",
+                MaxBattleCount = 5,
+                NextCandidateId = 1,
+            });
+            await db.SaveChangesAsync();
+        }
+
+        using var scope = factory.Services.CreateScope();
+        var builder = scope.ServiceProvider.GetRequiredService<IMatchContextBuilder>();
+        var ctx = await builder.BuildForTwoPickAsync(vid);
+
+        // DefaultLoadoutConfig.ShippedDefaults: EmblemId=100000000, DegreeId=300003
+        Assert.That(ctx.EmblemId, Is.EqualTo("100000000"));
+        Assert.That(ctx.DegreeId, Is.EqualTo("300003"));
+        Assert.That(ctx.CharaId, Is.EqualTo("1"));    // falls back to ClassId
+    }
+}