docs(kernel): document the evidenced contract members added for #4251

The IDataEngine and IMetadataService contract references enumerate the
interface surface member by member, so the two additions (driver registry
lookups; registerInMemory) were stale-by-omission — flagged by the docs
drift check on #4321. Both interface listings and the per-member sections
now match packages/spec/src/contracts.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AWW5xEALZNU5VBXfGyWPGz

Author: claude

content/docs/kernel/contracts/data-engine.mdx

@@ -52,6 +52,10 @@ export interface IDataEngine {
 
   // Raw Command Escape Hatch (optional)
   execute?(command: any, options?: Record<string, any>): Promise<any>;
+
+  // Driver Registry (optional — engines that own named drivers)
+  getDefaultDriverName?(): string | undefined;
+  getDriverByName?(name: string): IDataDriver | undefined;
 }
 ```
 
@@ -365,6 +369,21 @@ const result = await engine.execute?.(
 );
 ```
 
+### getDefaultDriverName / getDriverByName (Driver Registry)
+
+Look up the engine's registered drivers. Only engines that own a named-driver
+registry implement these (ObjectQL does; test fakes and remote/virtual engines
+need not) — always probe with `?.`:
+
+```typescript
+const driverName = engine.getDefaultDriverName?.();
+const driver = driverName ? engine.getDriverByName?.(driverName) : undefined;
+```
+
+This is the surface the runtime uses to re-register the default driver as a
+`driver.<name>` kernel service, which is where `os migrate` and serve's storage
+detection locate drivers.
+
 ---
 
 ## Error Codes

content/docs/kernel/contracts/metadata-service.mdx

@@ -30,6 +30,7 @@ definition for a type.
 export interface IMetadataService {
   // Core CRUD (by type + name)
   register(type: string, name: string, data: unknown): Promise<void>;
+  registerInMemory?(type: string, name: string, data: unknown): void;
   get(type: string, name: string): Promise<unknown | undefined>;
   list(type: string): Promise<unknown[]>;
   unregister(type: string, name: string): Promise<void>;
@@ -166,6 +167,26 @@ await metadataService.unregister('object', 'project');
 To inspect what would break before removing an item, call `getDependents('object', 'project')` — it returns the items that reference this one.
 </Callout>
 
+### registerInMemory
+
+Optional. Seeds an item into the in-memory registry **only** — never persisted
+to the DB store, never announced to `watch` subscribers. This is the boot-time
+path for source-control-owned artefacts that must be *listable* without
+creating DB drift — e.g. code-defined datasources (`origin: 'code'`), which is
+how the default datasource appears in Setup → Datasources:
+
+```typescript
+metadataService.registerInMemory?.('datasource', 'default', {
+  name: 'default',
+  label: 'Default',
+  driver: 'sqlite',
+  origin: 'code',
+});
+```
+
+Callers that mutate metadata mid-run want `register`, which persists and
+announces.
+
 ---
 
 ## Bulk Operations