docs: clarify cache behavior across navigations

[aurorascharff]

Summary

• document stable SWR keys so remounts can reuse browser data
• keep repeat and browser-back assertions in the three Skills, linking to the existing preserved-state testing guide
• keep production rig templates focused on build, server, and test execution
• document the Route Handler limitation for browser-private cache output once in the use cache: private API reference
• require Partial Prefetching tests to target the final destination rather than a redirect

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

[github-actions]

Tests Passed

Commit: b42896b

[SapanMozammel]

Good clarification on return-navigation testing patterns and webServer lifecycle.