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/Importers/BattlePassImporter.cs

@@ -0,0 +1,37 @@
+using System.Text.Json;
+using Microsoft.EntityFrameworkCore;
+using SVSim.Bootstrap.Models.Seed;
+using SVSim.Database;
+using SVSim.Database.Models;
+
+namespace SVSim.Bootstrap.Importers;
+
+/// <summary>
+/// Idempotent upsert of battle-pass level rows from <c>seeds/battle-pass-levels.json</c>.
+/// Per-level <c>reward_data</c> blob preserved verbatim (shape varies per level).
+/// </summary>
+public class BattlePassImporter
+{
+    public async Task<int> ImportAsync(SVSimDbContext context, string seedDir)
+    {
+        var seed = SeedLoader.LoadList<BattlePassLevelSeed>(Path.Combine(seedDir, "battle-pass-levels.json"));
+        if (seed.Count == 0) return 0;
+
+        var existing = await context.BattlePassLevels.ToDictionaryAsync(e => e.Id);
+        int created = 0, updated = 0;
+        foreach (var s in seed)
+        {
+            if (s.Id == 0) continue;
+            var entry = existing.TryGetValue(s.Id, out var ex) ? ex : new BattlePassLevelEntry { Id = s.Id };
+            entry.RewardData = s.RewardData.ValueKind == JsonValueKind.Undefined
+                ? "{}"
+                : JsonSerializer.Serialize(s.RewardData);
+            if (ex is null) { context.BattlePassLevels.Add(entry); existing[s.Id] = entry; created++; }
+            else updated++;
+        }
+
+        await context.SaveChangesAsync();
+        Console.WriteLine($"[BattlePassImporter] +{created}/~{updated}");
+        return created + updated;
+    }
+}