docs: clarify cache behavior across navigations - #98590

Summary

• document stable SWR cache keys so remounts and repeat navigations can reuse browser data
• clarify how to scope use cache: private identities and handle browser-held output after writes
• teach all three instant() test rigs to distinguish fresh-destination absence from hidden DOM retained during repeat or browser-back navigation
• align the Partial Prefetching rigs with the shared server-lifecycle and conclusive-result rules from docs: add guide for optimizing the static shell #97737
• ensure Partial Prefetching adoption tests target canonical destinations instead of redirecting links

This is stacked on #97737 and can be retargeted to canary after it merges.

Verification

• focused Prettier check
• focused Alex language lint
• git diff --check
• repository pre-commit lint-staged checks