perf: offload per-import localized patch to worker + cache FurnitureData parse

Per single furni import, patchLocalizedFurniDataEntries ran for every
language and did a full 100k-entry JSON.parse + scan + JSON.stringify on the
main thread. That CPU spike is now offloaded to the translation worker
(init/prepare/finalize reuses the same pure core helpers), so importing
never blocks the event loop. The main thread only does async file I/O and the
network LibreTranslate calls. Falls back to the same helpers on the main
thread if no worker is available.

Also cache readFurniData() by file mtime+size so the 100k-entry JSON is
parsed once per change instead of on every import/fix/reconcile read
(invalidated on write).

src/lib/services/furni-data.ts

@@ -199,10 +199,34 @@ export async function withFurniDataLock<T>(fn: () => Promise<T>): Promise<T> {
 	}
 }
 
+let cachedFurniData: {
+	path: string;
+	mtimeMs: number;
+	size: number;
+	data: Record<string, unknown>;
+} | null = null;
+
 export async function readFurniData(): Promise<Record<string, unknown>> {
 	const target = await getFurnitureDataReadPath();
+	const stat = await fs.stat(target);
+	if (
+		cachedFurniData &&
+		cachedFurniData.path === target &&
+		cachedFurniData.mtimeMs === stat.mtimeMs &&
+		cachedFurniData.size === stat.size
+	) {
+		// File unchanged since last parse: skip the (expensive) full JSON.parse.
+		return cachedFurniData.data;
+	}
 	const raw = await fs.readFile(/*turbopackIgnore: true*/ target, "utf-8");
-	return JSON.parse(raw);
+	const data = JSON.parse(raw) as Record<string, unknown>;
+	cachedFurniData = {
+		path: target,
+		mtimeMs: stat.mtimeMs,
+		size: stat.size,
+		data,
+	};
+	return data;
 }
 
 export async function writeFurniData(
@@ -216,6 +240,9 @@ export async function writeFurniData(
 		await fs.writeFile(/*turbopackIgnore: true*/ tmp, payload, "utf-8");
 		await fs.rename(/*turbopackIgnore: true*/ tmp, target);
 	}
+	// The file just changed: force the next read to reparse instead of serving
+	// a possibly-mutated cached object.
+	cachedFurniData = null;
 }
 
 export function buildFurniEntry(params: {