perf(native-chat): a publish behind a delivered commit reads nothing

Each commit now delivers itself, so the publish a provider frame still sends
afterwards found every reader caught up but still read rows and rebuilt the
timeline for each one. A caught-up reader now skips the read.

src/main/native-chat/agent-session-wire/structured-agent-session-append-delivery.test.ts

@@ -192,6 +192,11 @@ describe('an open chat receives each row once', () => {
   it('when the provider frame that wrote it also publishes', async () => {
     const pane = liveReader()
     const sink = providerSink()
+    const journal = host['sessions'].get(SESSION)?.journal
+    if (!journal) {
+      throw new Error('the attached chat has no journal')
+    }
+    const readSince = vi.spyOn(journal, 'readSince')
 
     sink.appendItem(
       { provider: 'orca', clientMessageId: 'streamed' },
@@ -201,6 +206,8 @@ describe('an open chat receives each row once', () => {
     await host.flushStreamedEvents(SESSION)
 
     expect(pane.received().statuses).toEqual(['streamed row'])
+    // The frame's publish finds the reader caught up and reads no rows: streaming stays one read.
+    expect(readSince).toHaveBeenCalledOnce()
   })
 
   it('when a writer publishes the row it appended', async () => {

src/main/native-chat/agent-session-wire/structured-agent-session-subscribers.ts

@@ -15,6 +15,7 @@ import {
   type AgentSessionSubscribeEvent,
   type AgentSessionTurnActivity
 } from '../../../shared/agent-session-wire'
+import { sameJournalCursor } from '../agent-session-journal/journal-cursor'
 import type { AgentSessionJournal } from '../agent-session-journal/journal-store'
 import { emptyAgentSessionBatch } from './agent-session-empty-batch'
 import {
@@ -213,6 +214,11 @@ export class AgentSessionSubscribers {
       ? this.activityField(subscriber.sessionId).activity
       : undefined
     const publishedActivity = activity !== undefined ? activity : checkpointActivity
+    // Caught up, so there are no rows to read: every publish behind a commit's own delivery.
+    if (!journal.isReadOnly && sameJournalCursor(subscriber.cursor, journal.cursor())) {
+      this.emitCaughtUp(subscriber, hostNow, emitCheckpoint, backgroundTasks, publishedActivity)
+      return
+    }
     const readPage = createAgentSessionCatchUpReader(journal)
     while (true) {
       const result = readPage({
@@ -239,20 +245,7 @@ export class AgentSessionSubscribers {
       const page = result.page
       const advanced = page.window.nextCursor.sequence > subscriber.cursor.sequence
       if (!advanced) {
-        const commandsChanged =
-          this.hooks.readCommands !== undefined &&
-          (this.hooks.readCommands(subscriber.sessionId) ?? null) !== subscriber.commands
-        if (emitCheckpoint || publishedActivity !== undefined || commandsChanged) {
-          this.emit(subscriber, {
-            type: 'batch',
-            sessionId: subscriber.sessionId,
-            batch: emptyAgentSessionBatch(page.window.nextCursor),
-            fence: subscriber.fence,
-            hostNow,
-            ...(backgroundTasks !== undefined ? { backgroundTasks } : {}),
-            ...(publishedActivity !== undefined ? { activity: publishedActivity } : {})
-          })
-        }
+        this.emitCaughtUp(subscriber, hostNow, emitCheckpoint, backgroundTasks, publishedActivity)
         return
       }
       this.emit(subscriber, {
@@ -276,6 +269,29 @@ export class AgentSessionSubscribers {
     }
   }
 
+  private emitCaughtUp(
+    subscriber: Subscriber,
+    hostNow: number,
+    emitCheckpoint: boolean,
+    backgroundTasks: AgentSessionBackgroundTaskState | null | undefined,
+    activity: AgentSessionTurnActivity | null | undefined
+  ): void {
+    const commandsChanged =
+      this.hooks.readCommands !== undefined &&
+      (this.hooks.readCommands(subscriber.sessionId) ?? null) !== subscriber.commands
+    if (emitCheckpoint || activity !== undefined || commandsChanged) {
+      this.emit(subscriber, {
+        type: 'batch',
+        sessionId: subscriber.sessionId,
+        batch: emptyAgentSessionBatch(subscriber.cursor),
+        fence: subscriber.fence,
+        hostNow,
+        ...(backgroundTasks !== undefined ? { backgroundTasks } : {}),
+        ...(activity !== undefined ? { activity } : {})
+      })
+    }
+  }
+
   private now = (): number => this.hooks.now?.() ?? Date.now()
 
   private isActive = (subscriber: Subscriber): boolean =>