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/runtime/opfs/snapshot-store.js

@@ -0,0 +1,63 @@
+import { loadTextFile, saveTextFile } from "./file-service.js";
+import { sessionSnapshotPath } from "./path-model.js";
+
+export const SESSION_SNAPSHOT_FORMAT = "linuxcnc-wasm-session-snapshot";
+export const SESSION_SNAPSHOT_VERSION = 1;
+
+function assertPlainObject(value, label) {
+  if (value === null || typeof value !== "object" || Array.isArray(value)) {
+    throw new Error(`${label} must be a plain object.`);
+  }
+}
+
+export function createSessionSnapshot(sessionId, payload, options = {}) {
+  sessionSnapshotPath(sessionId, options.filename);
+  assertPlainObject(payload, "snapshot payload");
+  if (options.metadata !== undefined) {
+    assertPlainObject(options.metadata, "snapshot metadata");
+  }
+
+  return {
+    format: SESSION_SNAPSHOT_FORMAT,
+    version: SESSION_SNAPSHOT_VERSION,
+    sessionId,
+    createdAt: options.createdAt ?? new Date().toISOString(),
+    metadata: options.metadata ?? {},
+    payload,
+  };
+}
+
+export function validateSessionSnapshot(snapshot, sessionId) {
+  assertPlainObject(snapshot, "session snapshot");
+  if (snapshot.format !== SESSION_SNAPSHOT_FORMAT) {
+    throw new Error(`Unsupported session snapshot format: ${snapshot.format}`);
+  }
+  if (snapshot.version !== SESSION_SNAPSHOT_VERSION) {
+    throw new Error(`Unsupported session snapshot version: ${snapshot.version}`);
+  }
+  if (snapshot.sessionId !== sessionId) {
+    throw new Error(`Session snapshot id mismatch: ${snapshot.sessionId}`);
+  }
+  assertPlainObject(snapshot.metadata, "snapshot metadata");
+  assertPlainObject(snapshot.payload, "snapshot payload");
+  return snapshot;
+}
+
+export async function saveSessionSnapshot(sessionId, payload, options = {}) {
+  const snapshot = createSessionSnapshot(sessionId, payload, options);
+  const path = sessionSnapshotPath(sessionId, options.filename);
+  await saveTextFile(path, `${JSON.stringify(snapshot, null, 2)}\n`, options.storage);
+  return snapshot;
+}
+
+export async function loadSessionSnapshot(sessionId, options = {}) {
+  const path = sessionSnapshotPath(sessionId, options.filename);
+  const text = await loadTextFile(path, options.storage);
+  let snapshot;
+  try {
+    snapshot = JSON.parse(text);
+  } catch (error) {
+    throw new Error(`Invalid session snapshot JSON: ${error.message}`);
+  }
+  return validateSessionSnapshot(snapshot, sessionId);
+}