feat(FN-2121): introduce structured logging in plugin loader

- Add a reusable createLogger utility in @fusion/core for prefixed log/warn/error output
- Replace plugin-loader console logging with structured logger calls across load, reload, stop, and hook paths
- Route plugin-scoped logger methods through createLogger, including debug gating on DEBUG=plugins
- Add regression tests that mock logger.js and verify key structured log emissions and error logging flows

packages/core/src/logger.ts

@@ -0,0 +1,47 @@
+/**
+ * Lightweight structured logger for the `@fusion/core` package.
+ *
+ * Usage:
+ * ```ts
+ * import { createLogger } from "./logger.js";
+ * const log = createLogger("my-module");
+ * log.log("hello");   // → console.error("[my-module] hello")
+ * log.warn("oops");   // → console.warn("[my-module] oops")
+ * log.error("fail");  // → console.error("[my-module] fail")
+ * ```
+ *
+ * Core subsystems should use this utility rather than calling `console.*`
+ * directly so diagnostics stay consistent and easy to suppress/match in tests.
+ */
+
+export interface Logger {
+  log(message: string, ...args: unknown[]): void;
+  warn(message: string, ...args: unknown[]): void;
+  error(message: string, ...args: unknown[]): void;
+}
+
+/**
+ * Create a structured logger that prefixes every message with `[prefix]`.
+ *
+ * @param prefix - Short subsystem name, e.g. "plugin-loader".
+ * @returns A `Logger` whose output is prefixed and sent to stderr for normal
+ *          logs and errors. Keeping logs off stdout prevents command/test
+ *          output consumers from receiving Fusion execution chatter.
+ */
+export function createLogger(prefix: string): Logger {
+  const tag = `[${prefix}]`;
+  return {
+    log(message: string, ...args: unknown[]) {
+      console.error(`${tag} ${message}`, ...args);
+    },
+    warn(message: string, ...args: unknown[]) {
+      console.warn(`${tag} ${message}`, ...args);
+    },
+    error(message: string, ...args: unknown[]) {
+      console.error(`${tag} ${message}`, ...args);
+    },
+  };
+}
+
+/** Logger for the plugin loader subsystem. */
+export const pluginLoaderLog = createLogger("plugin-loader");