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/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);
+}