CultCache TS: one function derives a definition's schema identity

Soul F1 (hold on 98999997..6e47710e): the cache stamped records with
schemaId ?? schemaName ?? type, while the CultNet registry and CultMesh's
schema descriptor used schemaId ?? type. A definition that set only a
schema name was stamped one id by the cache and bound under another by the
registry, so its records could land on a peer as a different type.

- schemaIdentityOf(definition) returns the schema id, schema name, schema
  version and every id the definition answers to (its own first). It is
  exported, and it is the only place that derivation lives.
- The cache's catalog entry is built from it; nothing else changes here.

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

packages/cultcache-ts/src/cult-cache.ts

@@ -1,5 +1,6 @@
 import { decode, encode } from "@msgpack/msgpack";
 
+import { schemaIdentityOf } from "./document";
 import { SchemaConflictError } from "./store-format";
 import type {
   AnyCultCacheDocumentDefinition,
@@ -706,8 +707,7 @@ export class CultCache {
   }
 
   #createCatalogEntry(definition: AnyCultCacheDocumentDefinition): CultCacheSchemaCatalogEntry {
-    const schemaName = definition.schemaName ?? definition.type;
-    const schemaVersion = definition.schemaVersion ?? `${schemaName}.v1`;
+    const { schemaId, schemaName, schemaVersion, compatibleSchemaIds } = schemaIdentityOf(definition);
     const canonicalSchemaJson = definition.canonicalSchemaJson ?? JSON.stringify({
       schemaName,
       schemaVersion,
@@ -724,10 +724,6 @@ export class CultCache {
           isName: member.isName === true,
         })),
     });
-    const schemaId = definition.schemaId ?? schemaName;
-    const compatibleSchemaIds = [
-      ...new Set([schemaId, ...(definition.compatibleSchemaIds ?? [])]),
-    ];
 
     return {
       schemaId,
@@ -735,7 +731,7 @@ export class CultCache {
       schemaVersion,
       contentHash: definition.contentHash ?? schemaId,
       canonicalSchemaJson,
-      compatibleSchemaIds,
+      compatibleSchemaIds: [...compatibleSchemaIds],
       members: [...(definition.members ?? [])]
         .sort((left, right) => left.slot - right.slot)
         .map((member) => ({

packages/cultcache-ts/src/document.ts

@@ -69,6 +69,27 @@ export function defineDocumentType<TSchema extends CultCacheSchema>(
   return Object.freeze({ ...definition });
 }
 
+// The identity a definition's records carry: the schema id they are stamped with, the schema name and version, and every id
+// the definition answers to (its own first). Every CultCache runtime surface and every CultNet or CultMesh layer over it
+// derives a definition's identity here and nowhere else, so a record's id means the same type wherever it is read.
+export interface CultCacheSchemaIdentity {
+  readonly schemaId: string;
+  readonly schemaName: string;
+  readonly schemaVersion: string;
+  readonly compatibleSchemaIds: readonly string[];
+}
+
+export function schemaIdentityOf(definition: AnyCultCacheDocumentDefinition): CultCacheSchemaIdentity {
+  const schemaName = definition.schemaName ?? definition.type;
+  const schemaId = definition.schemaId ?? schemaName;
+  return {
+    schemaId,
+    schemaName,
+    schemaVersion: definition.schemaVersion ?? `${schemaName}.v1`,
+    compatibleSchemaIds: [...new Set([schemaId, ...(definition.compatibleSchemaIds ?? [])])],
+  };
+}
+
 export function defineDocumentRegistry<
   TDefinitions extends readonly AnyCultCacheDocumentDefinition[],
 >(...definitions: TDefinitions): CultCacheDocumentRegistry<TDefinitions> {

packages/cultcache-ts/src/index.ts

@@ -1,6 +1,6 @@
 export { CultCache, CultCacheBuilder } from "./cult-cache";
 export { inspectCultCacheBytes } from "./cult-cache-inspector";
-export { defineDocumentRegistry, defineDocumentType } from "./document";
+export { defineDocumentRegistry, defineDocumentType, schemaIdentityOf, type CultCacheSchemaIdentity } from "./document";
 export { SingleFileMessagePackBackingStore } from "./single-file-messagepack-backing-store";
 export { SchemaConflictError, StoreUnreadableError } from "./store-format";
 export type {