Guard OPFS preview-cache staging

wasm-port/tests/opfs/node/verify_file_service.mjs

@@ -111,6 +111,14 @@ function assertSessionSnapshotStoragePath(sessionId, path, filename = "snapshot.
   );
 }
 
+function assertPreviewCacheStoragePath(cacheKey, path, filename = "preview.json") {
+  assert.equal(
+    path,
+    `linuxcnc/preview-cache/${cacheKey}/${filename}`,
+    `${cacheKey}/${filename} preview cache OPFS path drift`,
+  );
+}
+
 assert.equal(await getOpfsRoot(storage), root);
 
 assert.equal(normalizeOpfsPath("linuxcnc/machines/xyzab.ini"), "linuxcnc/machines/xyzab.ini");
@@ -122,7 +130,12 @@ assertGcodeProgramStoragePath(
   "custom-fixture.ngc",
   gcodeProgramPath("custom-fixture.ngc"),
 );
-assert.equal(previewCachePath("fixture"), "linuxcnc/preview-cache/fixture/preview.json");
+assertPreviewCacheStoragePath("fixture", previewCachePath("fixture"));
+assertPreviewCacheStoragePath(
+  "fixture",
+  previewCachePath("fixture", "custom-preview.json"),
+  "custom-preview.json",
+);
 assertSessionSnapshotStoragePath(
   "session-1",
   sessionSnapshotPath("session-1"),