接上一轮，按接续文件继续执行

结论：Three.js 程序预览和刀具执行显示已收口，公网 HTTP/IP 下 Save/Restore Session 已支持 memory-fallback 降级并通过 node/browser gate。

web-rtcp-5axis-sim-plan/app/src/runtime/five-axis-session.js

@@ -88,20 +88,22 @@ export function validateFiveAxisSessionSnapshot(snapshot, sessionId) {
 export async function saveFiveAxisSessionSnapshot(sessionId, payload, options = {}) {
   const snapshot = createFiveAxisSessionSnapshot(sessionId, payload, options);
   const path = sessionSnapshotPath(sessionId, options.filename);
-  await saveTextFile(path, `${JSON.stringify(snapshot, null, 2)}\n`, options.storage);
-  return { snapshot, path };
+  const storage = resolveSessionStorage(options);
+  await saveTextFile(path, `${JSON.stringify(snapshot, null, 2)}\n`, storage.storage);
+  return { snapshot, path, storageMode: storage.mode };
 }
 
 export async function loadFiveAxisSessionSnapshot(sessionId, options = {}) {
   const path = sessionSnapshotPath(sessionId, options.filename);
-  const text = await loadTextFile(path, options.storage);
+  const storage = resolveSessionStorage(options);
+  const text = await loadTextFile(path, storage.storage);
   let snapshot;
   try {
     snapshot = JSON.parse(text);
   } catch (error) {
     throw new Error(`Invalid five-axis session snapshot JSON: ${error.message}`);
   }
-  return { snapshot: validateFiveAxisSessionSnapshot(snapshot, sessionId), path };
+  return { snapshot: validateFiveAxisSessionSnapshot(snapshot, sessionId), path, storageMode: storage.mode };
 }
 
 export function restoreFiveAxisSessionState(snapshot) {
@@ -138,6 +140,7 @@ export function restoreFiveAxisSessionState(snapshot) {
 export function createMemorySessionStorage(seed = {}) {
   const files = new Map(Object.entries(seed));
   return {
+    apiName: "web-rtcp-5axis-memory-session-storage",
     files,
     async getDirectory() {
       return createDirectoryHandle(files, []);
@@ -145,6 +148,35 @@ export function createMemorySessionStorage(seed = {}) {
   };
 }
 
+let browserMemorySessionStorage = null;
+
+function resolveSessionStorage(options = {}) {
+  if (options.storage) {
+    return {
+      storage: options.storage,
+      mode: options.storageMode || storageModeFor(options.storage),
+    };
+  }
+  const browserStorage = globalThis.navigator?.storage;
+  if (browserStorage?.getDirectory) {
+    return {
+      storage: browserStorage,
+      mode: "opfs",
+    };
+  }
+  browserMemorySessionStorage ??= createMemorySessionStorage();
+  return {
+    storage: browserMemorySessionStorage,
+    mode: "memory-fallback",
+  };
+}
+
+function storageModeFor(storage) {
+  return storage?.apiName === "web-rtcp-5axis-memory-session-storage"
+    ? "memory"
+    : "custom";
+}
+
 function validateFiveAxisSessionPayload(payload) {
   assertPlainObject(payload, "five-axis session payload");
   if (payload.apiName !== "web-rtcp-5axis-session-payload") {