Guard OPFS path helper staging

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

@@ -119,6 +119,18 @@ function assertPreviewCacheStoragePath(cacheKey, path, filename = "preview.json"
   );
 }
 
+function assertMachineFileStoragePaths(machineId, paths) {
+  assert.deepEqual(
+    paths,
+    {
+      ini: `linuxcnc/machines/${machineId}/machine.ini`,
+      toolTable: `linuxcnc/machines/${machineId}/tool.tbl`,
+      parameters: `linuxcnc/machines/${machineId}/linuxcnc.var`,
+    },
+    `${machineId} machine file OPFS path drift`,
+  );
+}
+
 assert.equal(await getOpfsRoot(storage), root);
 
 assert.equal(normalizeOpfsPath("linuxcnc/machines/xyzab.ini"), "linuxcnc/machines/xyzab.ini");
@@ -145,12 +157,8 @@ assertSessionSnapshotStoragePath(
   sessionSnapshotPath("session-1", "custom-snapshot.json"),
   "custom-snapshot.json",
 );
-assert.deepEqual(defaultMachinePaths("xyzab-tdr"), {
-  ini: "linuxcnc/machines/xyzab-tdr/machine.ini",
-  toolTable: "linuxcnc/machines/xyzab-tdr/tool.tbl",
-  parameters: "linuxcnc/machines/xyzab-tdr/linuxcnc.var",
-});
-assert.deepEqual(machineFilePaths("xyzab-tdr"), defaultMachinePaths("xyzab-tdr"));
+assertMachineFileStoragePaths("xyzab-tdr", defaultMachinePaths("xyzab-tdr"));
+assertMachineFileStoragePaths("xyzab-tdr", machineFilePaths("xyzab-tdr"));
 
 const snapshotPayload = {
   files: {
@@ -653,6 +661,10 @@ assert.throws(
   () => sessionSnapshotPath("session-1", "nested/snapshot.json"),
   /Invalid session snapshot filename/,
 );
+assert.throws(
+  () => previewCachePath("fixture", "nested/preview.json"),
+  /Invalid preview cache filename/,
+);
 assert.throws(
   () => gcodeProgramPath("../escape.ngc"),
   /Invalid G-code filename/,