fix(stats): kept worker pool for one-file syncs

Keyed the serial-parser branch off the caller-requested worker count instead of the per-pool clamp, so a one-file sync still fans out to workers when the caller (or default) asked for more than one.

Fixes #3733

packages/stats/src/aggregator.ts

@@ -241,14 +241,16 @@ export async function syncAllSessions(opts?: SyncOptions): Promise<{ processed:
 		report(sessionFile);
 	};
 
-	const poolSize = Math.max(1, Math.min(files.length, opts?.workers ?? defaultWorkerCount()));
-	if (poolSize === 1) {
+	const requestedWorkers = Math.max(1, Math.floor(opts?.workers ?? defaultWorkerCount()));
+	if (requestedWorkers === 1) {
 		for (const sessionFile of files) {
 			await processFile(sessionFile, parseSessionFile);
 		}
 		return { processed: totalProcessed, files: filesProcessed };
 	}
 
+	const poolSize = Math.min(files.length, requestedWorkers);
+
 	const handles: WorkerHandle[] = [];
 	for (let i = 0; i < poolSize; i++) handles.push(spawnWorker());
 

packages/stats/test/sync-serial.test.ts

@@ -88,4 +88,15 @@ describe("stats sync serial mode", () => {
 		expect(overall.totalRequests).toBe(1);
 		expect(workerSpy).not.toHaveBeenCalled();
 	});
+
+	it("spawns a worker pool when callers explicitly request workers: 2 with a single file", async () => {
+		await writeSessionFile();
+		const workerProbe = new Error("worker probe");
+		const workerSpy = vi.spyOn(globalThis, "Worker").mockImplementation(() => {
+			throw workerProbe;
+		});
+
+		await expect(syncAllSessions({ workers: 2 })).rejects.toBe(workerProbe);
+		expect(workerSpy).toHaveBeenCalled();
+	});
 });