perf: optimize persist/load P1 hot paths

tests/graph-persistence.mjs

@@ -3239,6 +3239,166 @@ result = {
   );
 }
 
+{
+  const chatId = "chat-idb-direct-delta-prebuilt-persist-snapshot";
+  const baseGraph = createMeaningfulGraph(chatId, "direct-delta-base");
+  const runtimeGraph = createMeaningfulGraph(chatId, "direct-delta-after");
+  const baseSnapshot = buildSnapshotFromGraph(baseGraph, {
+    chatId,
+    revision: 7,
+  });
+  const persistSnapshot = buildSnapshotFromGraph(runtimeGraph, {
+    chatId,
+    revision: 8,
+    baseSnapshot,
+  });
+  const directDelta = buildPersistDelta(baseSnapshot, persistSnapshot, {
+    useNativeDelta: false,
+  });
+  const harness = await createGraphPersistenceHarness({
+    chatId,
+    globalChatId: chatId,
+    chatMetadata: {
+      integrity: "meta-idb-direct-delta-prebuilt-persist-snapshot",
+    },
+    indexedDbSnapshot: baseSnapshot,
+  });
+  harness.api.setCurrentGraph(runtimeGraph);
+  harness.api.setGraphPersistenceState({
+    loadState: "loaded",
+    chatId,
+    revision: 8,
+    lastPersistedRevision: 0,
+    writesBlocked: false,
+  });
+
+  const originalBuildSnapshotFromGraph = harness.runtimeContext.buildSnapshotFromGraph;
+  let buildSnapshotCallCount = 0;
+  harness.runtimeContext.buildSnapshotFromGraph = (...args) => {
+    buildSnapshotCallCount += 1;
+    return originalBuildSnapshotFromGraph(...args);
+  };
+
+  const result = await harness.api.saveGraphToIndexedDb(chatId, runtimeGraph, {
+    revision: 8,
+    reason: "direct-delta-prebuilt-persist-snapshot-save",
+    scheduleCloudUpload: false,
+    persistDelta: directDelta,
+    persistSnapshot,
+  });
+
+  assert.equal(result.saved, true);
+  assert.equal(
+    buildSnapshotCallCount,
+    0,
+    "direct-delta 且已提供 persistSnapshot 时不应再次构建 snapshot",
+  );
+  assert.equal(result.snapshot?.meta?.revision, 8);
+  assert.equal(harness.api.getIndexedDbSnapshot()?.meta?.revision, 8);
+}
+
+{
+  const chatId = "chat-indexeddb-probe-empty-early-return";
+  const persistedSnapshot = {
+    meta: { revision: 0, chatId },
+    nodes: [],
+    edges: [],
+    tombstones: [],
+    state: {
+      lastProcessedFloor: -1,
+      extractionCount: 0,
+    },
+  };
+  const harness = await createGraphPersistenceHarness({
+    chatId,
+    globalChatId: chatId,
+    chatMetadata: {
+      integrity: "meta-indexeddb-probe-empty-early-return",
+    },
+    indexedDbSnapshot: persistedSnapshot,
+  });
+  harness.runtimeContext.__globalChatId = chatId;
+  harness.runtimeContext.__chatContext.chatId = chatId;
+  harness.api.setChatContext({
+    ...harness.api.getChatContext(),
+    chatId,
+    chatMetadata: {
+      integrity: "meta-indexeddb-probe-empty-early-return",
+    },
+  });
+  harness.api.setCurrentGraph(
+    createMeaningfulGraph(chatId, "probe-empty-runtime-current"),
+  );
+  harness.api.setGraphPersistenceState({
+    loadState: "loaded",
+    chatId,
+    revision: 1,
+    lastPersistedRevision: 1,
+    storagePrimary: "indexeddb",
+    storageMode: "indexeddb",
+    writesBlocked: false,
+  });
+
+  const originalCreateDb = harness.runtimeContext.BmeChatManager.prototype._createDb;
+  let exportSnapshotCalls = 0;
+  let exportProbeCalls = 0;
+  harness.runtimeContext.BmeChatManager.prototype._createDb = function(dbChatId = "") {
+    const baseDb = originalCreateDb.call(this, dbChatId);
+    return {
+      ...baseDb,
+      async exportSnapshot() {
+        exportSnapshotCalls += 1;
+        return await baseDb.exportSnapshot();
+      },
+      async exportSnapshotProbe() {
+        exportProbeCalls += 1;
+        const snapshot = harness.api.getIndexedDbSnapshotForChat(dbChatId) || {
+          meta: { revision: 0, chatId: String(dbChatId || "") },
+          state: { lastProcessedFloor: -1, extractionCount: 0 },
+          nodes: [],
+          edges: [],
+          tombstones: [],
+        };
+        return {
+          meta: {
+            ...(snapshot.meta || {}),
+            chatId: String(dbChatId || ""),
+            revision: Number(snapshot?.meta?.revision || 0),
+            nodeCount: Array.isArray(snapshot?.nodes) ? snapshot.nodes.length : 0,
+            edgeCount: Array.isArray(snapshot?.edges) ? snapshot.edges.length : 0,
+            tombstoneCount: Array.isArray(snapshot?.tombstones)
+              ? snapshot.tombstones.length
+              : 0,
+          },
+          state: {
+            lastProcessedFloor: Number(snapshot?.state?.lastProcessedFloor ?? -1),
+            extractionCount: Number(snapshot?.state?.extractionCount ?? 0),
+          },
+          nodes: [],
+          edges: [],
+          tombstones: [],
+          __stBmeProbeOnly: true,
+          __stBmeTombstonesOmitted: true,
+        };
+      },
+    };
+  };
+
+  const result = await harness.api.loadGraphFromIndexedDb(chatId, {
+    source: "probe-empty-early-return",
+    attemptIndex: 0,
+  });
+
+  assert.equal(result.loaded, false);
+  assert.equal(exportProbeCalls, 1);
+  assert.equal(
+    exportSnapshotCalls,
+    0,
+    "empty/probe 早退应在 probe 阶段终止，而不是继续全量导出 snapshot",
+  );
+  harness.runtimeContext.BmeChatManager.prototype._createDb = originalCreateDb;
+}
+
 {
   const harness = await createGraphPersistenceHarness({
     chatId: "chat-pending-persist-retry",