Clear (abort) the platform's canary: drop the version row and soft-remove every machine.

#108 O7 (lmcp-win#192): for windows this is NOT a promote. The public Windows serve picks the highest win-% row by released_at, held back only by the serve-time exclusion on the active canary_release row — so dropping that row would publish the already-registered win-<v> to the whole fleet. Stamp the canary's win row withdrawn_at BEFORE clearing, so the public serve keeps skipping it; only an explicit promote (which does not withdraw) serves it. A later set-version of the same version clears the stamp (re-canarying un-withdraws). darwin needs none of this: its public serve is gated on is_latest, which a canary row never carries until promote flips it.