test(terminal): pin the per-PTY ACK-silence skip in the write-off lane

Review ablation found the skip at accounting.ts unguarded: deleting it left
all 13 heal cases green. The existing sibling test does not reach it, because
its live pane ACKs everything main managed to send — the per-PTY send cap
means sentChars equals ackedChars, so zero debt makes the earlier guard skip
that pty first.

A PARTIAL ack is what reaches it: the live pane keeps real debt while having
answered a moment ago, which is exactly the state the skip protects. Without
it that pane is written off in the same pass as its wedged sibling, because
the heal is legitimately open and nothing else distinguishes them.

src/main/ipc/pty-delivery-health-heal.test.ts

@@ -406,6 +406,39 @@ describe('registerPtyHandlers', () => {
       vi.useRealTimers()
     }
   })
+  it('spares a PTY still round-tripping ACKs while a sibling is written off', () => {
+    vi.useFakeTimers()
+    const warnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {})
+    try {
+      const provider = installObservableDaemonTestProvider()
+      registerPtyHandlers(mainWindow as never)
+      provider.emitData('pty-wedged', 'x'.repeat(600 * 1024))
+      provider.emitData('pty-live', 'x'.repeat(600 * 1024))
+      vi.advanceTimersByTime(2)
+      for (let index = 0; index < 400; index++) {
+        vi.advanceTimersByTime(1)
+      }
+
+      // A PARTIAL ACK, so the live pane keeps real debt rather than settling to zero. That is
+      // the state the per-PTY silence skip exists for: the heal is legitimately open because
+      // the wedged pane is silent, and only the skip stops the same pass from writing off a
+      // pane that answered a moment ago and would have repaid the rest itself.
+      getPtyAckDataListener()(null, { id: 'pty-live', processedChars: 256 * 1024 })
+
+      const healed = reportRendererDeliveryState({
+        receivedCharsByPty: { 'pty-wedged': 512 * 1024 },
+        processedCharsByPty: {},
+        parkedCharsByPty: { 'pty-wedged': 512 * 1024 },
+        heal: true,
+        rendererPtyDataListenerCount: 1
+      })
+
+      expect(healed.writtenOff?.map((entry) => entry.id)).toEqual(['pty-wedged'])
+    } finally {
+      warnSpy.mockRestore()
+      vi.useRealTimers()
+    }
+  })
   it('reports per-PTY debt so the renderer can tell which pane is wedged', async () => {
     vi.useFakeTimers()
     const mockProc = createMockProc()