perf(terminal): keep retained-tail fallback lazy

src/main/runtime/rpc/methods/terminal.ts

@@ -2682,7 +2682,6 @@ export const TERMINAL_METHODS: RpcAnyMethod[] = [
                   'initial-snapshot'
                 )
               : null
-          const retainedTail = read.tail.length > 0 ? `${read.tail.join('\r\n')}\r\n` : ''
           const snapshotPublication = sendSnapshotFrames(
             (opcode, payload) => sendFrame(request.streamId, opcode, payload),
             {
@@ -2703,7 +2702,9 @@ export const TERMINAL_METHODS: RpcAnyMethod[] = [
               // tail, is a genuinely empty pane and must not be reported as a failure.
               unavailable:
                 serialized || read.tail.length > 0 ? undefined : 'no-serializable-buffer',
-              data: serializedContent?.data ?? retainedTail
+              data:
+                serializedContent?.data ??
+                (read.tail.length > 0 ? `${read.tail.join('\r\n')}\r\n` : '')
             }
           )
           const replacement = stream.sourceRangeReplacement

src/main/runtime/rpc/terminal-multiplex-initial-snapshot-fallback.test.ts

@@ -142,6 +142,19 @@ describe('terminal multiplex initial snapshot fallback', () => {
     expect(snapshot.start.unavailable).toBeUndefined()
   })
 
+  it('does not rebuild the retained tail when serialized content is available', async () => {
+    const tail = ['retained line']
+    const tailJoin = vi.spyOn(tail, 'join')
+    const snapshot = await subscribeSnapshot({
+      connectionId: 'conn-serialized-snapshot-tail',
+      serialized: { data: 'serialized screen', cols: 120, rows: 40 },
+      tail
+    })
+
+    expect(snapshot.data).toBe('serialized screen')
+    expect(tailJoin).not.toHaveBeenCalled()
+  })
+
   it('reports unavailable when neither serialized data nor a retained tail exists', async () => {
     const snapshot = await subscribeSnapshot({
       connectionId: 'conn-null-snapshot-no-tail',