Guard OPFS snapshot staging

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

@@ -103,6 +103,14 @@ function assertGcodeProgramStoragePath(filename, path) {
   );
 }
 
+function assertSessionSnapshotStoragePath(sessionId, path, filename = "snapshot.json") {
+  assert.equal(
+    path,
+    `linuxcnc/sessions/${sessionId}/${filename}`,
+    `${sessionId}/${filename} session snapshot OPFS path drift`,
+  );
+}
+
 assert.equal(await getOpfsRoot(storage), root);
 
 assert.equal(normalizeOpfsPath("linuxcnc/machines/xyzab.ini"), "linuxcnc/machines/xyzab.ini");
@@ -115,10 +123,14 @@ assertGcodeProgramStoragePath(
   gcodeProgramPath("custom-fixture.ngc"),
 );
 assert.equal(previewCachePath("fixture"), "linuxcnc/preview-cache/fixture/preview.json");
-assert.equal(sessionSnapshotPath("session-1"), "linuxcnc/sessions/session-1/snapshot.json");
-assert.equal(
+assertSessionSnapshotStoragePath(
+  "session-1",
+  sessionSnapshotPath("session-1"),
+);
+assertSessionSnapshotStoragePath(
+  "session-1",
   sessionSnapshotPath("session-1", "custom-snapshot.json"),
-  "linuxcnc/sessions/session-1/custom-snapshot.json",
+  "custom-snapshot.json",
 );
 assert.deepEqual(defaultMachinePaths("xyzab-tdr"), {
   ini: "linuxcnc/machines/xyzab-tdr/machine.ini",
@@ -182,6 +194,10 @@ await saveSessionSnapshot("session-1", snapshotPayload, {
   createdAt: "2026-06-08T00:00:00.000Z",
   metadata: { source: "node-smoke" },
 });
+assertSessionSnapshotStoragePath(
+  "session-1",
+  sessionSnapshotPath("session-1"),
+);
 assert.deepEqual(await loadSessionSnapshot("session-1", { storage }), snapshot);
 const customSnapshot = await saveSessionSnapshot("session-1", snapshotPayload, {
   storage,
@@ -189,6 +205,11 @@ const customSnapshot = await saveSessionSnapshot("session-1", snapshotPayload, {
   createdAt: "2026-06-08T00:00:00.000Z",
   metadata: { source: "node-smoke-custom" },
 });
+assertSessionSnapshotStoragePath(
+  "session-1",
+  sessionSnapshotPath("session-1", "custom-snapshot.json"),
+  "custom-snapshot.json",
+);
 assert.equal(
   customSnapshot.metadata.source,
   "node-smoke-custom",