CultNet C#: a second type for one schema id is refused, typed

Soul F5. F7 (operator, 2026-09-30): "Schema aliasing sounds like a really
bad idea, I support retiring it." CultNetDocumentRegistry.Register wrote
both maps unconditionally, so the last binding for a schema id won.

- Register refuses a binding whose type already holds another schema id,
  or whose schema id another type holds, with the reference runtime's
  CultSchemaConflictException: the new binding's schema id, both document
  types, an empty record key. Nothing is bound on refusal. Binding a type
  to the schema id it holds again replaces the binding. This matches Rust
  (fc3f3ee5) and Kotlin.
- Tests: both claimant shapes in both orders with two bindings ahead of
  the holder (F6/M11), the first binding intact afterwards, and a same-type
  rebind replacing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/GameCult.Networking/CultNetDocumentRegistry.cs

@@ -165,10 +165,31 @@ namespace GameCult.Networking
         }
 
         /// <summary>
-        /// Registers a document binding.
+        /// Registers a document binding. One binding owns a document type and a schema id: a binding claiming a type or a schema
+        /// id another binding already holds is refused with <see cref="CultSchemaConflictException"/>, naming the new binding's
+        /// schema id and both document types, and nothing is bound. Binding a type to the schema id it already holds replaces
+        /// that binding.
         /// </summary>
         public CultNetDocumentRegistry Register(CultNetDocumentBinding binding)
         {
+            var holder =
+                _bindingsByType.TryGetValue(binding.DocumentType, out var sameType) &&
+                !string.Equals(sameType.SchemaId, binding.SchemaId, StringComparison.Ordinal)
+                    ? sameType
+                    : _bindingsBySchemaId.TryGetValue(binding.SchemaId, out var sameSchema) &&
+                      sameSchema.DocumentType != binding.DocumentType
+                        ? sameSchema
+                        : null;
+            if (holder != null)
+            {
+                throw new CultSchemaConflictException(
+                    $"CultNet schema id \"{holder.SchemaId}\" is already bound to type \"{holder.DocumentType.FullName}\"; " +
+                    $"type \"{binding.DocumentType.FullName}\" (schema id \"{binding.SchemaId}\") cannot also be bound.",
+                    binding.SchemaId,
+                    new[] { holder.DocumentType.FullName ?? holder.DocumentType.Name, binding.DocumentType.FullName ?? binding.DocumentType.Name },
+                    string.Empty);
+            }
+
             _bindingsBySchemaId[binding.SchemaId] = binding;
             _bindingsByType[binding.DocumentType] = binding;
             return this;

tests/GameCult.Networking.Tests/CultNetDocumentRegistryConflictTests.cs

@@ -0,0 +1,91 @@
+#nullable enable
+using System;
+using GameCult.Caching;
+using MessagePack;
+using NUnit.Framework;
+
+namespace GameCult.Networking.Tests
+{
+    // F7 (operator, 2026-09-30): "Schema aliasing sounds like a really bad idea, I support retiring it." One binding owns a
+    // document type and a schema id, in the reference runtime as in Rust, Kotlin and TypeScript.
+    public sealed class CultNetDocumentRegistryConflictTests
+    {
+        private static CultNetDocumentBinding Bind<T>(string? schemaId = null) where T : class =>
+            CultNetDocumentBinding.ForDocument<T>(CultDocumentRegistry.Shared, schemaId);
+
+        // Two bindings stand ahead of the holder, so a conflict check that looks only at the first holder misses it.
+        private static CultNetDocumentRegistry RegistryHolding(CultNetDocumentBinding holder) =>
+            new CultNetDocumentRegistry(CultDocumentRegistry.Shared)
+                .Register(Bind<ConflictFillerA>())
+                .Register(Bind<ConflictFillerB>())
+                .Register(holder);
+
+        [Test]
+        public void A_second_type_for_one_schema_id_is_refused_typed_in_either_order()
+        {
+            var first = Bind<ConflictNote>("f5.shared");
+            var second = Bind<ConflictOtherNote>("f5.shared");
+            foreach (var (held, claimant) in new[] { (first, second), (second, first) })
+            {
+                var registry = RegistryHolding(held);
+                var error = Assert.Throws<CultSchemaConflictException>(() => registry.Register(claimant))!;
+                Assert.That(error.SchemaId, Is.EqualTo("f5.shared"));
+                Assert.That(error.SchemaNames, Is.EqualTo(new[] { held.DocumentType.FullName, claimant.DocumentType.FullName }));
+                Assert.That(error.RecordKey, Is.EqualTo(string.Empty));
+                Assert.That(error.Message, Does.Contain($"type \"{claimant.DocumentType.FullName}\""));
+                // Nothing was bound: the schema id still resolves to the first type, and the claimant's type to nothing.
+                Assert.That(registry.GetBySchemaId("f5.shared"), Is.SameAs(held));
+                Assert.That(registry.GetByDocumentType(claimant.DocumentType), Is.Null);
+            }
+        }
+
+        [Test]
+        public void A_type_bound_to_one_schema_id_cannot_be_bound_to_another()
+        {
+            var own = Bind<ConflictNote>();
+            var registry = RegistryHolding(own);
+            var error = Assert.Throws<CultSchemaConflictException>(() => registry.Register(Bind<ConflictNote>("f5.elsewhere")))!;
+            Assert.That(error.SchemaId, Is.EqualTo("f5.elsewhere"));
+            Assert.That(registry.GetBySchemaId("f5.elsewhere"), Is.Null);
+            Assert.That(registry.GetByDocumentType(typeof(ConflictNote)), Is.SameAs(own));
+        }
+
+        [Test]
+        public void Binding_a_type_to_the_schema_id_it_holds_again_replaces_the_binding()
+        {
+            var registry = RegistryHolding(Bind<ConflictNote>());
+            var again = Bind<ConflictNote>();
+            registry.Register(again);
+            Assert.That(registry.GetByDocumentType(typeof(ConflictNote)), Is.SameAs(again));
+            Assert.That(registry.GetBySchemaId(again.SchemaId), Is.SameAs(again));
+        }
+
+        [CultDocument("f5.conflict-note", "f5.conflict-note.v1")]
+        [MessagePackObject(AllowPrivate = true)]
+        public sealed class ConflictNote
+        {
+            [Key(0)] public string Body = string.Empty;
+        }
+
+        [CultDocument("f5.conflict-other-note", "f5.conflict-other-note.v1")]
+        [MessagePackObject(AllowPrivate = true)]
+        public sealed class ConflictOtherNote
+        {
+            [Key(0)] public string Body = string.Empty;
+        }
+
+        [CultDocument("f5.conflict-filler-a", "f5.conflict-filler-a.v1")]
+        [MessagePackObject(AllowPrivate = true)]
+        public sealed class ConflictFillerA
+        {
+            [Key(0)] public string Body = string.Empty;
+        }
+
+        [CultDocument("f5.conflict-filler-b", "f5.conflict-filler-b.v1")]
+        [MessagePackObject(AllowPrivate = true)]
+        public sealed class ConflictFillerB
+        {
+            [Key(0)] public string Body = string.Empty;
+        }
+    }
+}