Use `ResponseCompleteness` enum for segment cache write paths
Replace the `isResponsePartial` boolean in
`writeDynamicRenderResponseIntoCache` and `writeSeedDataIntoCache` with
a three-state `ResponseCompleteness` enum that distinguishes:
- `Partial`: segments have dynamic holes (static stage responses,
PPRRuntime prefetches with `~` marker, postponed responses)
- `Complete`: all segments are complete, but the head may still be
partial per the server's flag (Full/LoadingBoundary prefetches)
- `FullyStatic`: server explicitly marked the response as fully static
(marker byte `#`) — both segments and head are complete
The boolean conflated two independent concerns: segment partiality and
whether the server's `isHeadPartial` flag can be overridden. This caused
Full prefetches (which are complete but may have partial heads) to
incorrectly override `isHeadPartial` to `false`, breaking loading
boundary display for pages with dynamic metadata.
