Make legacy PPR paths explicit (#96753)
## Summary
Make the remaining legacy PPR implementation explicit by renaming the
render capability, prerender store, component, and tracking helper
around `React.unstable_postpone`.
`isLegacyPPR` is only enabled for route-level PPR without Cache
Components. The existing `prerender-ppr` work-unit discriminator and
rendering behavior are preserved, making this deprecated path easier to
audit and eventually remove.
