docs(review): note why a pump Send racing program exit is safe

Review asked whether the pump can Send after the Bubble Tea program exits (select picking a queued msg after done closes). It can — and it's a no-op by construction: tea.Program.Send is a context-guarded select and the msgs channel is never closed, so a post-exit Send returns immediately rather than panicking or blocking. Recorded at the pump so the next reader doesn't re-derive it from the tea source.