feat(arena-colosseum): 2-pick + curated deck sources (phase 3)

Closes the family. arena-colosseum 10/16 → 15/16 zero stubs (the 16th —
finish_load — is dead per project_dead_battle_endpoints).

* 2-Pick draft lift onto ArenaColosseumController:
  - get_candidate_classes samples from ArenaTwoPickConfig.AllowedClassIds
    and persists the slate onto the run.
  - class_choose accepts class_id XOR chaos_id; both populate run.ClassId,
    chaos branch stores ChaosId for replay.
  - get_candidate_cards is the idempotent draft-resume snapshot.
  - card_choose appends both cards from the picked pair, advances turn 1..15.
  - Pool override: ArenaTwoPickCardPoolService gets a non-breaking
    GeneratePickSetsForTurn(..., poolCardSetIds) overload; Colosseum routes
    pass ColosseumSeasonConfig.PoolCardSetIds (falls back to challenge →
    rotation when empty).
* Curated-deck schema: ColosseumHofDeck / ColosseumWindFallDeck /
  ColosseumAvatarDeck — three identical tables (separate per per-pool
  operational lifecycle), unique on DeckNo. Migration AddColosseumCuratedDecks.
* IColosseumCuratedDeck interface + a generic ColosseumCuratedDeckImporterBase<T>
  with three concrete subclasses (HOF / WindFall / Avatar), registered in
  Bootstrap. Seed files ship empty.
* 6 curated endpoints on ArenaColosseumController: get_{hof|windfall|avatar}_deck_list
  return BARE-ARRAY data per spec; register_{hof|windfall|avatar}_deck share
  one generic RegisterCuratedAsync<T> dispatcher. Cross-pool register
  rejected via per-pool lookup. Curated register has no is_published flag
  (constructed-only) — clears the run's flag for state consistency.
* Tests: 5 draft HTTP tests + 11 curated-deck HTTP tests (4 parameterized
  3 ways across HOF/WindFall/Avatar + a cross-pool isolation test + an
  is_published clear test). Existing TwoPick service tests updated for the
  new pool overload. Full suite: 1347/1347.

Phase 3 ship gate met. Branch ready for merge.

SVSim.EmulatedEntrypoint/Models/Dtos/Responses/ArenaColosseum/GetCandidateCardsResponse.cs

@@ -0,0 +1,27 @@
+using System.Text.Json.Serialization;
+using MessagePack;
+using SVSim.EmulatedEntrypoint.Models.Dtos.Common.ArenaTwoPick;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.Responses.ArenaColosseum;
+
+/// <summary>
+/// <c>POST /arena_colosseum/get_candidate_cards</c>. Idempotent draft-resume — server emits
+/// the current snapshot for the active run plus the pending pair offer. The Common
+/// <c>DeckInfoDto</c>/<c>CandidatePairDto</c>/<c>ClassInfoDto</c> shapes are shared with
+/// arena-two-pick and arena-competition per spec.
+/// </summary>
+[MessagePackObject]
+public sealed class GetCandidateCardsResponse
+{
+    [JsonPropertyName("deck_info")] [Key("deck_info")]
+    public DeckInfoDto DeckInfo { get; set; } = new();
+
+    [JsonPropertyName("candidate_card_list")] [Key("candidate_card_list")]
+    public List<CandidatePairDto> CandidateCardList { get; set; } = new();
+
+    [JsonPropertyName("leader_skin_id")] [Key("leader_skin_id")]
+    public long? LeaderSkinId { get; set; }
+
+    [JsonPropertyName("class_info")] [Key("class_info")]
+    public ClassInfoDto ClassInfo { get; set; } = new();
+}

SVSim.EmulatedEntrypoint/Models/Dtos/Responses/ArenaColosseum/GetCandidateClassesResponse.cs

@@ -0,0 +1,41 @@
+using System.Text.Json.Serialization;
+using MessagePack;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.Responses.ArenaColosseum;
+
+/// <summary>
+/// <c>POST /arena_colosseum/get_candidate_classes</c>. Two mutually-exclusive sub-shapes —
+/// Normal 2-pick emits <c>class_id_1/2/3</c>; Chaos emits <c>chaos_id_1/2/3</c> +
+/// <c>chaos_info</c>. <c>WhenWritingNull</c> strips the inactive branch so the wire matches
+/// the spec exactly.
+/// </summary>
+[MessagePackObject]
+public sealed class GetCandidateClassesResponse
+{
+    [JsonPropertyName("class_id_1")] [Key("class_id_1")]
+    public int? ClassId1 { get; set; }
+
+    [JsonPropertyName("class_id_2")] [Key("class_id_2")]
+    public int? ClassId2 { get; set; }
+
+    [JsonPropertyName("class_id_3")] [Key("class_id_3")]
+    public int? ClassId3 { get; set; }
+
+    [JsonPropertyName("chaos_id_1")] [Key("chaos_id_1")]
+    public int? ChaosId1 { get; set; }
+
+    [JsonPropertyName("chaos_id_2")] [Key("chaos_id_2")]
+    public int? ChaosId2 { get; set; }
+
+    [JsonPropertyName("chaos_id_3")] [Key("chaos_id_3")]
+    public int? ChaosId3 { get; set; }
+
+    [JsonPropertyName("selected_chaos_id")] [Key("selected_chaos_id")]
+    public int? SelectedChaosId { get; set; }
+
+    [JsonPropertyName("selected_class_id")] [Key("selected_class_id")]
+    public int? SelectedClassId { get; set; }
+
+    [JsonPropertyName("selected_leader_skin_id")] [Key("selected_leader_skin_id")]
+    public long? SelectedLeaderSkinId { get; set; }
+}