feat(upstream): Add dynamic per-request upstream provider and integrate into registries

ts/core/interfaces.core.ts

@@ -3,7 +3,7 @@
  */
 
 import type * as plugins from '../plugins.js';
-import type { IProtocolUpstreamConfig } from '../upstream/interfaces.upstream.js';
+import type { IUpstreamProvider } from '../upstream/interfaces.upstream.js';
 import type { IAuthProvider } from './interfaces.auth.js';
 import type { IStorageHooks } from './interfaces.storage.js';
 
@@ -89,8 +89,6 @@ export interface IProtocolConfig {
   enabled: boolean;
   basePath: string;
   features?: Record<string, boolean>;
-  /** Upstream registry configuration for proxying/caching */
-  upstream?: IProtocolUpstreamConfig;
 }
 
 /**
@@ -113,6 +111,13 @@ export interface IRegistryConfig {
    */
   storageHooks?: IStorageHooks;
 
+  /**
+   * Dynamic upstream configuration provider.
+   * Called per-request to resolve which upstream registries to use.
+   * Use StaticUpstreamProvider for simple static configurations.
+   */
+  upstreamProvider?: IUpstreamProvider;
+
   oci?: IProtocolConfig;
   npm?: IProtocolConfig;
   maven?: IProtocolConfig;