按建议，继续完成后续工作

结论：已新增 OPFS session snapshot store，提供通用 JSON 包络、格式校验和保存/读取往返验证；该层只处理 host-side 持久化，不定义 CNC 机床状态语义。

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

@@ -15,6 +15,12 @@ import {
   sessionSnapshotPath,
   toolTablePath,
 } from "../../../runtime/opfs/path-model.js";
+import {
+  createSessionSnapshot,
+  loadSessionSnapshot,
+  saveSessionSnapshot,
+  validateSessionSnapshot,
+} from "../../../runtime/opfs/snapshot-store.js";
 
 class MockFileHandle {
   constructor(name) {
@@ -86,6 +92,26 @@ assert.deepEqual(defaultMachinePaths("xyzab-tdr"), {
   parameters: "linuxcnc/machines/xyzab-tdr/linuxcnc.var",
 });
 
+const snapshotPayload = {
+  files: {
+    ini: machineIniPath("xyzab-tdr"),
+    toolTable: toolTablePath("xyzab-tdr"),
+  },
+};
+const snapshot = createSessionSnapshot("session-1", snapshotPayload, {
+  createdAt: "2026-06-08T00:00:00.000Z",
+  metadata: { source: "node-smoke" },
+});
+assert.deepEqual(snapshot, {
+  format: "linuxcnc-wasm-session-snapshot",
+  version: 1,
+  sessionId: "session-1",
+  createdAt: "2026-06-08T00:00:00.000Z",
+  metadata: { source: "node-smoke" },
+  payload: snapshotPayload,
+});
+assert.equal(validateSessionSnapshot(snapshot, "session-1"), snapshot);
+
 await saveTextFile(
   "linuxcnc/machines/xyzab.ini",
   "[EMC]\nMACHINE = opfs-smoke\n",
@@ -96,6 +122,13 @@ assert.equal(
   "[EMC]\nMACHINE = opfs-smoke\n",
 );
 
+await saveSessionSnapshot("session-1", snapshotPayload, {
+  storage,
+  createdAt: "2026-06-08T00:00:00.000Z",
+  metadata: { source: "node-smoke" },
+});
+assert.deepEqual(await loadSessionSnapshot("session-1", { storage }), snapshot);
+
 await assert.rejects(
   () => loadTextFile("linuxcnc/machines/missing.ini", storage),
   /missing file/,
@@ -108,6 +141,15 @@ assert.throws(
   () => machineIniPath("../escape"),
   /Invalid machine id/,
 );
+await saveTextFile(sessionSnapshotPath("bad-json"), "{", storage);
+await assert.rejects(
+  () => loadSessionSnapshot("bad-json", { storage }),
+  /Invalid session snapshot JSON/,
+);
+await assert.rejects(
+  () => loadSessionSnapshot("missing-session", { storage }),
+  /missing directory|missing file/,
+);
 await assert.rejects(
   () => getOpfsRoot({}),
   /OPFS is not available/,