refactor(bootstrap): add 7 load-index importers (excluding card lists)

Stage 9B of the bootstrap-seed-refactor: add per-domain importer classes
that consume the load-index seed split produced in Stage 9A.

New importers (each in its own file under SVSim.Bootstrap/Importers/):
- RotationConfigImporter: writes Rotation/Challenge/MyRotationSchedule
  GameConfig sections (atomic UpsertSection<T> pattern, copied private-static
  from GlobalsImporter so this importer stands alone post-9C).
- MyRotationImporter: settings + abilities (extractor pre-joins on rotation_id).
- AvatarAbilityImporter: per-leader_skin_id ability rows.
- ArenaSeasonImporter: singleton (Id=1) Take Two arena season.
- BattlePassImporter: per-level reward blobs.
- DailyLoginBonusImporter: per-bonus-id campaign blobs.
- PreReleaseInfoImporter: singleton (Id=1) pre-release window.

Seed DTOs under SVSim.Bootstrap/Models/Seed/ mirror the seed JSON via
[JsonPropertyName] snake_case. Raw-JSON columns (reward_data, format_info,
etc.) use JsonElement on the seed and JsonSerializer.Serialize in the
importer.

Tests: 7 new happy-path tests in LoadIndexImporterTests.cs (idempotency
covered by BattlePass spot-check). Full suite: 382/382 passing (375 + 7).

NOT modifying in this stage: GlobalsImporter.cs (Stage 9C strips the old
methods), Program.cs (Stage 9C wires up all 9 importers), SVSimTestFactory
(Stage 9C). Double-writing on bootstrap is expected and OK during 9B.

SVSim.Bootstrap/Models/Seed/BattlePassLevelSeed.cs

@@ -0,0 +1,11 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace SVSim.Bootstrap.Models.Seed;
+
+/// <summary>Mirrors <c>seeds/battle-pass-levels.json</c>. <c>reward_data</c> preserved verbatim.</summary>
+public sealed class BattlePassLevelSeed
+{
+    [JsonPropertyName("id")] public int Id { get; set; }
+    [JsonPropertyName("reward_data")] public JsonElement RewardData { get; set; }
+}