feat: Implement Plugin Marketplace Manager and Client

- Added PluginMarketplaceManager to coordinate marketplace reads and manage installed plugins.
- Introduced PluginMarketplaceClient for fetching marketplace metadata.
- Implemented storage management for installed plugins using MemoryStorage.
- Added methods for installing, uninstalling, and enabling/disabling plugins.
- Created type definitions for PluginMarketplaceManager options and installed plugin records.
- Added support for fetching full marketplace catalog and individual plugin metadata.
- Included error handling for fetch operations and validation of plugin data.
- Updated index.browser.ts to export new modules and types.

dist/PluginMarketplaceClient.d.ts

@@ -0,0 +1,53 @@
+import type { PluginIndex, PluginMetadata } from './types.js';
+/**
+ * Minimal response shape required by marketplace client.
+ */
+export interface PluginMarketplaceResponse {
+    ok: boolean;
+    status: number;
+    json(): Promise<unknown>;
+}
+/**
+ * Fetch function used by marketplace client.
+ */
+export type PluginMarketplaceFetch = (input: string, init?: Record<string, unknown>) => Promise<PluginMarketplaceResponse>;
+/**
+ * Construction options for {@link PluginMarketplaceClient}.
+ */
+export interface PluginMarketplaceClientOptions {
+    /** URL of marketplace index JSON file. */
+    indexUrl: string;
+    /** Base URL used to resolve `${pluginId}.json` metadata files. */
+    baseUrl: string;
+    /** Optional fetch implementation. Falls back to global `fetch`. */
+    fetchFn?: PluginMarketplaceFetch;
+}
+/**
+ * Host-agnostic client for reading marketplace metadata.
+ */
+export declare class PluginMarketplaceClient {
+    private readonly indexUrl;
+    private readonly baseUrl;
+    private readonly fetchFn?;
+    constructor(options: PluginMarketplaceClientOptions);
+    /**
+     * Fetches and validates marketplace index.
+     */
+    fetchIndex(): Promise<PluginIndex>;
+    /**
+     * Fetches and validates single plugin metadata entry.
+     *
+     * @param pluginId - Identifier referenced by marketplace index.
+     */
+    fetchPlugin(pluginId: string): Promise<PluginMetadata>;
+    /**
+     * Fetches full marketplace catalog.
+     *
+     * Entries that fail fetch or validation get skipped so host can still render
+     * rest of catalog.
+     */
+    fetchPlugins(): Promise<PluginMetadata[]>;
+    private buildMetadataUrl;
+    private getFetchFn;
+}
+//# sourceMappingURL=PluginMarketplaceClient.d.ts.map