fix(merge): isolate ambiguous private exports

Keep one-member private exports source-local until exact message overlap proves they belong to the same chat. This prevents unrelated outgoing-only chats from losing messages when platform IDs collide.

packages/node-runtime/src/merger/__tests__/merger.test.ts

@@ -178,6 +178,46 @@ describe('buildMergedOutput', () => {
     assert.equal(new Set(result.chatLabData.messages.map((message) => message.platformMessageId)).size, 2)
   })
 
+  it('keeps ambiguous one-member private chats source-local without verified overlap', () => {
+    const first = createMockSource(
+      { name: 'Alice', platform: 'telegram', type: 'private' },
+      [{ platformId: 'owner' }],
+      [
+        {
+          platformMessageId: '1',
+          senderPlatformId: 'owner',
+          timestamp: 100,
+          type: 0,
+          content: 'first chat',
+        },
+      ]
+    )
+    const second = createMockSource(
+      { name: 'Bob', platform: 'telegram', type: 'private' },
+      [{ platformId: 'owner' }],
+      [
+        {
+          platformMessageId: '1',
+          senderPlatformId: 'owner',
+          timestamp: 200,
+          type: 0,
+          content: 'second chat',
+        },
+      ]
+    )
+    const sources = [
+      { source: first, filename: 'alice.json' },
+      { source: second, filename: 'bob.json' },
+    ]
+
+    const conflicts = checkConflictsFromSources(sources)
+    const result = buildMergedOutput(sources, 'Merged')
+
+    assert.equal(conflicts.totalMessages, 2)
+    assert.equal(result.chatLabData.messages.length, 2)
+    assert.equal(new Set(result.chatLabData.messages.map((message) => message.platformMessageId)).size, 2)
+  })
+
   it('deduplicates overlapping private exports when their observed members differ', () => {
     const meta: MergerSourceMeta = { name: 'Alice', platform: 'qq', type: 'private' }
     const sharedMessage: MergerInputMessage = {

packages/node-runtime/src/merger/index.ts

@@ -56,12 +56,15 @@ function createMessageScope(meta: MergerSourceMeta | null, members: MergerMember
 
   const memberIds = [...new Set(members.map((member) => member.platformId))].sort()
   if (memberIds.length > 0) {
-    if (type === 'private') return JSON.stringify([platform, type, 'members', memberIds])
-    return JSON.stringify([platform, type, 'name-members', meta?.name || '', memberIds])
+    if (type === 'private') {
+      if (memberIds.length >= 2) return JSON.stringify([platform, type, 'members', memberIds])
+    } else {
+      return JSON.stringify([platform, type, 'name-members', meta?.name || '', memberIds])
+    }
   }
 
-  // Without a stable group ID or participant set, sharing message IDs across
-  // sources could silently merge unrelated chats. Keep those source-local.
+  // A one-sided private export does not identify the other participant. Without
+  // a stable group ID or complete participant set, keep message IDs source-local.
   return JSON.stringify([platform, type, 'source', sourceIndex])
 }