Introduce named Logger interface in keeper.ts

Replaces 'ReturnType<typeof makeLogger>' with a small named interface so
simulateActivity's signature reads as (config, log: Logger) instead of a
type-derivation chain. Also makes it cleaner to substitute a mock logger
if simulateActivity is ever unit-tested.

No behavior change.

src/keeper.ts

@@ -41,11 +41,25 @@ const timestamp = (): string => {
 };
 
 /**
- * Build a verbose-gated logger. `info` is unconditional; `event` only fires
- * when the caller asked for verbose output. Returning a small object keeps
- * `simulateActivity` free of `if (verbose)` noise at every log site.
+ * Minimal log surface used by `simulateActivity` and `runKeeper`. Named so
+ * it can appear directly in function signatures (clearer than
+ * `ReturnType<typeof makeLogger>`) and so a test could substitute a fake
+ * implementation if needed.
+ *
+ *  - `info(msg)`  prints unconditionally.
+ *  - `event(msg)` prints only when `--verbose` / `verbose: true` is set.
  */
-function makeLogger(verbose: boolean): { info(msg: string): void; event(msg: string): void } {
+interface Logger {
+    info(msg: string): void;
+    event(msg: string): void;
+}
+
+/**
+ * Build a verbose-gated `Logger`. `info` is unconditional; `event` only
+ * fires when the caller asked for verbose output. Returning a small object
+ * keeps `simulateActivity` free of `if (verbose)` noise at every log site.
+ */
+function makeLogger(verbose: boolean): Logger {
     return {
         info: (msg: string): void => {
             console.log(msg);
@@ -79,7 +93,7 @@ function makeLogger(verbose: boolean): { info(msg: string): void; event(msg: str
  *     so the next idle-check sees "no movement" and doesn't misread the
  *     synthetic activity as the user returning.
  */
-async function simulateActivity(config: Config, log: ReturnType<typeof makeLogger>): Promise<void> {
+async function simulateActivity(config: Config, log: Logger): Promise<void> {
     const start: Point = await mouse.getPosition();
     const screenWidth: number = await screen.width();
     const screenHeight: number = await screen.height();