refactor(sync): replace message reloads with incremental tail sync

hub/src/store/messages.test.ts

@@ -24,6 +24,7 @@ describe('cancelQueuedMessage', () => {
         // Row should be gone from uninvoked list
         const remaining = store.messages.getUninvokedLocalMessages(session.id)
         expect(remaining).toHaveLength(0)
+        expect(store.messages.getMessageEpoch(session.id)).toBe(1)
     })
 
     it('already-invoked: returns status=invoked with full message row, row stays in DB', () => {
@@ -67,6 +68,7 @@ describe('cancelQueuedMessage', () => {
         if (second.status === 'cancelled') {
             expect(second.localId).toBeNull()
         }
+        expect(store.messages.getMessageEpoch(session.id)).toBe(1)
     })
 
     it('non-existent messageId: returns status=cancelled with localId=null', () => {
@@ -173,6 +175,75 @@ describe('cancelQueuedMessage', () => {
     })
 })
 
+describe('position pagination and structural epochs', () => {
+    it('returns rows strictly after a cursor and respects an inclusive snapshot head', () => {
+        const store = makeStore()
+        const session = makeSession(store, 'position-after')
+        const first = store.messages.addMessage(session.id, { text: 'first' })
+        const second = store.messages.addMessage(session.id, { text: 'second' })
+        const third = store.messages.addMessage(session.id, { text: 'third' })
+        store.messages.addMessage(session.id, { text: 'fourth' })
+
+        const rows = store.messages.getMessagesAfterPosition(
+            session.id,
+            10,
+            { at: first.invokedAt ?? first.createdAt, seq: first.seq },
+            { at: third.invokedAt ?? third.createdAt, seq: third.seq }
+        )
+
+        expect(rows.map((message) => message.id)).toEqual([second.id, third.id])
+    })
+
+    it('reports the newest composite position', () => {
+        const store = makeStore()
+        const session = makeSession(store, 'position-head')
+        const first = store.messages.addMessage(session.id, { text: 'first' })
+        const second = store.messages.addMessage(session.id, { text: 'second' })
+
+        expect(store.messages.getNewestMessagePosition(session.id)).toEqual({
+            at: second.invokedAt ?? second.createdAt,
+            seq: second.seq
+        })
+        expect(first.seq).toBeLessThan(second.seq)
+    })
+
+    it('bumps both epochs when session history is merged', () => {
+        const store = makeStore()
+        const source = makeSession(store, 'epoch-merge-source')
+        const target = makeSession(store, 'epoch-merge-target')
+        store.messages.addMessage(source.id, { text: 'source' })
+        store.messages.addMessage(target.id, { text: 'target' })
+
+        const result = store.messages.mergeSessionMessages(source.id, target.id)
+
+        expect(result.moved).toBe(1)
+        expect(store.messages.getMessageEpoch(source.id)).toBe(1)
+        expect(store.messages.getMessageEpoch(target.id)).toBe(1)
+    })
+
+    it('bumps the target epoch when a copied message lands behind the cached head', () => {
+        const store = makeStore()
+        const target = makeSession(store, 'epoch-copy-target')
+        const head = store.messages.addMessage(target.id, { text: 'head' })
+        const headPosition = {
+            at: head.invokedAt ?? head.createdAt,
+            seq: head.seq
+        }
+
+        const copied = store.messages.copyMessageToSession(target.id, {
+            content: { text: 'historical' },
+            createdAt: headPosition.at - 1_000,
+            localId: null,
+            invokedAt: headPosition.at - 1_000,
+            scheduledAt: null
+        })
+
+        expect(copied.seq).toBeGreaterThan(head.seq)
+        expect(store.messages.getMessagesAfterPosition(target.id, 10, headPosition)).toEqual([])
+        expect(store.messages.getMessageEpoch(target.id)).toBe(1)
+    })
+})
+
 describe('addMessage: scheduledAt invariants', () => {
     it('rejects scheduledAt without a localId — would silently invoke immediately', () => {
         const store = makeStore()