feat(config): wire /config/update + /config/update_challenge_config (closes family 4/4)

is_skip_gacha_effect, use_challenge_two_pick_premium_card, and
challenge_two_pick_sleeve_id all persist on ViewerInfo and round-trip
through /load/index.

The two challenge fields move from the global ChallengeConfig section to
ViewerInfo — they're viewer preferences, not server-wide policy. Premium
card defaults to 0; sleeve falls back to DefaultLoadoutConfig.SleeveId
(3000011) when unset. MatchContextBuilder's TK2 sleeve read switches to
the same viewer-scoped path.

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

SVSim.UnitTests/Services/MatchContextBuilderTests.cs

@@ -68,8 +68,9 @@ public class MatchContextBuilderTests
         // 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"));
+        // Sleeve falls back to DefaultLoadoutConfig.SleeveId when the viewer hasn't set
+        // a challenge-specific one via /config/update_challenge_config.
+        Assert.That(ctx.SleeveId, Is.EqualTo("3000011"));
     }
 
     [Test]