feat(upstream): Add upstream proxy/cache subsystem and integrate per-protocol upstreams

ts/core/interfaces.core.ts

@@ -3,6 +3,7 @@
  */
 
 import type * as plugins from '../plugins.js';
+import type { IProtocolUpstreamConfig } from '../upstream/interfaces.upstream.js';
 
 /**
  * Registry protocol types
@@ -86,6 +87,8 @@ export interface IProtocolConfig {
   enabled: boolean;
   basePath: string;
   features?: Record<string, boolean>;
+  /** Upstream registry configuration for proxying/caching */
+  upstream?: IProtocolUpstreamConfig;
 }
 
 /**