perf: offload SWF→Nitro conversion to a worker-thread pool

The synchronous convertSwfToNitro / extractIconFromSwf calls (pure CPU,
no DB/network) now run in a small worker pool so the main Node event
loop stays free during imports — the dominant import-time CPU spike is
moved off the request thread. The pool degrades gracefully to a
synchronous fallback if workers can't be created, and is skipped in the
test environment.

src/lib/services/furni-import.ts

@@ -34,8 +34,8 @@ import {
 } from "@/lib/services/import/core/download";
 import {
 	type ConversionResult,
-	convertSwfToNitro,
-	extractIconFromSwf,
+	convertSwfToNitroAsync,
+	extractIconFromSwfAsync,
 	parseNitroBundle,
 } from "@/lib/services/swf-to-nitro";
 import type { ImportSingleResult } from "@/types/furni";
@@ -766,7 +766,7 @@ export async function importSingleFurni(params: {
 	if (!iconOk && (swfOk || existsSync(/*turbopackIgnore: true*/ swfPath))) {
 		try {
 			const swfBuf = await fs.readFile(/*turbopackIgnore: true*/ swfPath);
-			const iconPng = extractIconFromSwf(swfBuf, baseClassname);
+			const iconPng = await extractIconFromSwfAsync(swfBuf, baseClassname);
 			if (iconPng) {
 				await fs.writeFile(/*turbopackIgnore: true*/ iconPath, iconPng);
 				iconOk = true;
@@ -795,7 +795,10 @@ export async function importSingleFurni(params: {
 		try {
 			if (existsSync(/*turbopackIgnore: true*/ swfPath)) {
 				const swfBuffer = await fs.readFile(/*turbopackIgnore: true*/ swfPath);
-				conversionResult = convertSwfToNitro(swfBuffer, baseClassname);
+				conversionResult = await convertSwfToNitroAsync(
+					swfBuffer,
+					baseClassname,
+				);
 				await fs.writeFile(
 					/*turbopackIgnore: true*/ nitroPath,
 					conversionResult.bundle,