Extracts parkReadyAt/parkIsReady as a tested pure fold per PLAN.md's phase
criterion; the rest is wording fixes — hitch bound, hazard width, forced-drain
scope, progressive CPU reclaim.
A rapid A→B→A flip now costs no plugin work: the restore cancels the still-pending
park outright. A park that has already written one FX carries a `partial` flag and is
superseded by its inverse rather than cancelled, so a half-parked chain is never stranded.
daniel