feat(pack): /pack/set_rotation_starter_class + selected_class_id read-back

Implements Task 9 from docs/superpowers/plans/2026-06-13-decomp-only-followups.md
— the pre-purchase commit endpoint the client fires from StarterClassSelectDialog
before opening a RotationStarterCardPack. Without this the per-class draw path
from yesterday's work is unreachable: the dialog blocks on the AJAX call and
never advances to /pack/open.

Wire surface:
- POST /pack/set_rotation_starter_class { pack_id, class_id } → EmptyData.
  Validates 1..8, rejects unknown packs (404), non-RS packs (400), and second
  commits (400 — one-shot per pack per spec).
- PackConfigDto carries selected_class_id (nullable, omitted via global
  WhenWritingNull policy when unset). Placement verified against decomp at
  PackInfoTask.cs:86 — it's on the parent PackConfig, NOT the child gacha as
  the original plan text had it.
- /pack/open cross-checks request.class_id against the persisted choice for
  RS packs; rejects 400 on missing-commit or class-mismatch so a tampered
  request can't swap pools after the choice is locked.

Storage:
- ViewerPackStarterClass owned entity; (ViewerId, PackId) unique index via the
  pattern from project_owned_collection_unique_index memory.
- Migration PackStarterClass — composite PK + FK cascade verified against
  fresh Postgres bootstrap.

Tests (9 new in PackControllerSetRotationStarterClassTests):
- Round-trip set → /pack/info shows selected_class_id.
- Field absent before any commit.
- Invalid class (0/9/-1/100) → 400.
- Already-chosen rejection.
- Non-RS pack / unknown pack rejection.
- /pack/open rejects no-prior-commit AND class-mismatch; succeeds when class
  matches the persisted choice.

Full suite: 1552 passed (1543 + 9).

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

SVSim.EmulatedEntrypoint/Controllers/PackController.cs

@@ -7,6 +7,7 @@ using SVSim.Database.Models;
 using SVSim.Database.Repositories.Pack;
 using SVSim.Database.Repositories.PackDrawTables;
 using SVSim.EmulatedEntrypoint.Models.Dtos;
+using SVSim.EmulatedEntrypoint.Models.Dtos.Common;
 using SVSim.EmulatedEntrypoint.Models.Dtos.Requests;
 using SVSim.EmulatedEntrypoint.Models.Dtos.Requests.Pack;
 using SVSim.EmulatedEntrypoint.Models.Dtos.Responses.Pack;
@@ -101,10 +102,20 @@ public class PackController : SVSimController
             .Select(c => new { c.FreeGachaCampaignId, c.LastClaimedAt, c.ClaimCount })
             .ToDictionaryAsync(x => x.FreeGachaCampaignId, x => (x.LastClaimedAt, x.ClaimCount));
 
+        // Per-viewer RotationStarter class choices keyed by pack id — populates
+        // selected_class_id on the parent PackConfigDto so the client's
+        // StarterClassSelectDialog can pre-select on revisit (PackInfoTask.cs:86 reads via
+        // TryGetValue, so absent-key for non-RS packs is intentional/safe).
+        var starterClassesByPackId = await _db.Viewers
+            .Where(v => v.Id == viewerId)
+            .SelectMany(v => v.PackStarterClasses)
+            .Select(s => new { s.PackId, s.ClassId })
+            .ToDictionaryAsync(x => x.PackId, x => x.ClassId);
+
         return new PackInfoResponse
         {
             PackConfigList = packs
-                .Select(p => ToDto(p, openCounts, ownedItemsByItemId, gachaPointBalancesByPackId, freeClaimsByCampaignId))
+                .Select(p => ToDto(p, openCounts, ownedItemsByItemId, gachaPointBalancesByPackId, freeClaimsByCampaignId, starterClassesByPackId))
                 .ToList(),
         };
     }
@@ -114,7 +125,8 @@ public class PackController : SVSimController
         IReadOnlyDictionary<int, ViewerPackOpenCount> openCounts,
         IReadOnlyDictionary<long, int> ownedItemsByItemId,
         IReadOnlyDictionary<int, int> gachaPointBalancesByPackId,
-        IReadOnlyDictionary<int, (DateTime LastClaimedAt, int ClaimCount)> freeClaimsByCampaignId)
+        IReadOnlyDictionary<int, (DateTime LastClaimedAt, int ClaimCount)> freeClaimsByCampaignId,
+        IReadOnlyDictionary<int, int> starterClassesByPackId)
     {
         int openCount = openCounts.TryGetValue(p.Id, out var oc) ? oc.OpenCount : 0;
 
@@ -204,6 +216,7 @@ public class PackController : SVSimController
             OpenCountLimit = p.OpenCountLimit,
             IsHide = p.IsHide ? 1 : 0,
             PackCategory = (int)p.PackCategory,
+            SelectedClassId = starterClassesByPackId.TryGetValue(p.Id, out var chosenClass) ? chosenClass : null,
             GachaPoint = gachaPointDto,
             IsPreRelease = p.IsPreRelease,
             ExistsPurchaseReward = false,
@@ -259,6 +272,43 @@ public class PackController : SVSimController
         };
     }
 
+    /// <summary>
+    /// /pack/set_rotation_starter_class — lock in the class for a RotationStarterCardPack.
+    /// One-shot per (viewer, pack); rejects the second attempt with 400. The client opens
+    /// the pack via /pack/open afterwards (see Wizard/StarterClassSelectDialog.cs which
+    /// chains this call into StarterPurchaseConfirmationDialog on 200).
+    /// </summary>
+    [HttpPost("set_rotation_starter_class")]
+    public async Task<ActionResult<EmptyResponse>> SetRotationStarterClass(
+        PackSetRotationStarterClassRequest request, CancellationToken ct)
+    {
+        if (!TryGetViewerId(out long viewerId)) return Unauthorized();
+
+        if (request.ClassId is < 1 or > 8)
+            return BadRequest(new { error = "invalid_class" });
+
+        var pack = await _packs.GetPack(request.PackId);
+        if (pack is null) return NotFound(new { error = "unknown_pack" });
+        if (pack.PackCategory != PackCategory.RotationStarterCardPack)
+            return BadRequest(new { error = "not_a_rotation_starter_pack" });
+
+        var viewer = await _db.Viewers
+            .Include(v => v.PackStarterClasses)
+            .FirstOrDefaultAsync(v => v.Id == viewerId, ct);
+        if (viewer is null) return Unauthorized();
+
+        if (viewer.PackStarterClasses.Any(s => s.PackId == request.PackId))
+            return BadRequest(new { error = "class_already_chosen" });
+
+        viewer.PackStarterClasses.Add(new ViewerPackStarterClass
+        {
+            PackId = request.PackId,
+            ClassId = request.ClassId,
+        });
+        await _db.SaveChangesAsync(ct);
+        return new EmptyResponse();
+    }
+
     [HttpPost("get_leader_skin_owned_status")]
     public ActionResult<Dictionary<string, Dictionary<string, object>>> GetLeaderSkinOwnedStatus(
         [FromBody] PackGetLeaderSkinOwnedStatusRequest _)
@@ -299,12 +349,25 @@ public class PackController : SVSimController
         // Rotation-starter packs require a class_id (1..8) to select the per-class card pool.
         // Reject if missing/out-of-range; reject class_id on non-RS packs so it can't sneak
         // through and pick up nothing (a class-filtered draw against a non-RS pack would
-        // return an empty pool and 500).
+        // return an empty pool and 500). For RS packs the client is expected to have already
+        // committed the choice via /pack/set_rotation_starter_class — cross-check that the
+        // request class matches the persisted one so a tampered request can't swap pools
+        // mid-flight.
         bool isRotationStarter = pack.PackCategory == PackCategory.RotationStarterCardPack;
         if (isRotationStarter)
         {
             if (request.ClassId is not int cid || cid < 1 || cid > 8)
                 return BadRequest(new { error = "class_id_required_for_rotation_starter" });
+            var persisted = await _db.Viewers
+                .Where(v => v.Id == viewerId)
+                .SelectMany(v => v.PackStarterClasses)
+                .Where(s => s.PackId == request.ParentGachaId)
+                .Select(s => (int?)s.ClassId)
+                .FirstOrDefaultAsync();
+            if (persisted is null)
+                return BadRequest(new { error = "rotation_starter_class_not_chosen" });
+            if (persisted != cid)
+                return BadRequest(new { error = "rotation_starter_class_mismatch" });
         }
         else if (request.ClassId.HasValue)
         {

SVSim.EmulatedEntrypoint/Models/Dtos/PackConfigDto.cs

@@ -118,4 +118,14 @@ public class PackConfigDto
     [JsonPropertyName("poster_type")]
     [Key("poster_type")]
     public int PosterType { get; set; }
+
+    /// <summary>
+    /// The viewer's already-chosen class for a RotationStarterCardPack (1..8). Null when the
+    /// viewer has not yet locked a choice via /pack/set_rotation_starter_class. Client at
+    /// PackInfoTask.cs:86 reads via <c>TryGetValue</c>, so a missing key is safe (don't emit
+    /// when null — global WhenWritingNull is sufficient here).
+    /// </summary>
+    [JsonPropertyName("selected_class_id")]
+    [Key("selected_class_id")]
+    public int? SelectedClassId { get; set; }
 }

SVSim.EmulatedEntrypoint/Models/Dtos/Requests/Pack/PackSetRotationStarterClassRequest.cs

@@ -0,0 +1,23 @@
+using System.Text.Json.Serialization;
+using MessagePack;
+using SVSim.EmulatedEntrypoint.Models.Dtos.Requests;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.Requests.Pack;
+
+/// <summary>
+/// Inbound /pack/set_rotation_starter_class body. Locks in the class choice for a
+/// RotationStarterCardPack before the user can open it. One-shot per (viewer, pack)
+/// per the spec; subsequent attempts return 400.
+/// See <c>Wizard/PackSetRotationStarterClassTask.cs</c>.
+/// </summary>
+[MessagePackObject]
+public class PackSetRotationStarterClassRequest : BaseRequest
+{
+    [JsonPropertyName("pack_id")]
+    [Key("pack_id")]
+    public int PackId { get; set; }
+
+    [JsonPropertyName("class_id")]
+    [Key("class_id")]
+    public int ClassId { get; set; }
+}