Give packs an identity so create can refuse missing deps and load in a stable order.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/HSchool.Server/Api/SchoolEndpoints.cs

@@ -80,6 +80,14 @@ internal static class SchoolEndpoints
                     Problem(StatusCodes.Status400BadRequest, "unknown-name-set", "The selected name set is not in the catalog."),
                 SchoolCreationError.UnknownNativeLanguage =>
                     Problem(StatusCodes.Status400BadRequest, "unknown-native-language", "The selected native language is not in that name set."),
+                SchoolCreationError.MissingMod =>
+                    Problem(
+                        StatusCodes.Status400BadRequest,
+                        "missing-mod",
+                        $"Mod '{outcome.MissingPackId}' is required but was not selected.",
+                        missing: outcome.MissingPackId),
+                SchoolCreationError.ModCycle =>
+                    Problem(StatusCodes.Status400BadRequest, "mod-cycle", "Selected mods have a cyclic dependency."),
                 _ => Results.Problem("Unknown error."),
             };
         })
@@ -468,7 +476,12 @@ internal static class SchoolEndpoints
         return true;
     }
 
-    private static IResult Problem(int statusCode, string code, string detail, StaffingOutcome? staffing = null)
+    private static IResult Problem(
+        int statusCode,
+        string code,
+        string detail,
+        StaffingOutcome? staffing = null,
+        string? missing = null)
     {
         var extensions = new Dictionary<string, object?> { ["code"] = code };
         if (staffing is not null)
@@ -479,6 +492,11 @@ internal static class SchoolEndpoints
             extensions["attempted"] = staffing.Attempted;
         }
 
+        if (missing is not null)
+        {
+            extensions["missing"] = missing;
+        }
+
         return Results.Problem(detail: detail, statusCode: statusCode, title: code, extensions: extensions);
     }
 }
@@ -492,10 +510,16 @@ internal sealed record CreateSchoolRequest(
     string? NameSetId,
     string? NativeLanguage);
 
-internal sealed record SchoolResponse(int Id, string Name, DateTime GameTime, bool Running, byte SpeedIndex)
+internal sealed record SchoolResponse(
+    int Id,
+    string Name,
+    DateTime GameTime,
+    bool Running,
+    byte SpeedIndex,
+    IReadOnlyList<string> ModIds)
 {
     public static SchoolResponse From(SchoolState school) =>
-        new(school.Id, school.Name, school.GameTime, school.Running, school.SpeedIndex);
+        new(school.Id, school.Name, school.GameTime, school.Running, school.SpeedIndex, school.ModIds);
 }
 
 /// <summary>Everything the main menu needs in one request.</summary>