Reframe the audience-cascade section in changelog-conventions — marketing up top, engineering as the narrative unfolds

The earlier framing read as 'marketing all the way down,' which fought the four-audience cascade the same section then described. Reframed to say what the cascade actually does: the lede / Why Care? / What's New? are marketing-shaped, the How it Works / Under the Hood / Files Touched are engineering-shaped, and both belong — what differs is sequence.