[Instant] Dev Tools toggle for Instant Navigation Mode (#90222)
Adds an "Instant Navigation Mode" toggle to the Dev Tools indicator
menu. When enabled, navigations only show the cached/prefetched state —
the same behavior as the instant navigation testing API, but toggled
from the dev tools UI instead of test code.
Gated behind `experimental.instantNavigationDevToolsToggle`.
This is a proof of concept — the UX needs more work before it's ready to
ship broadly (e.g. the toggle should be more discoverable, and the
locked state needs clearer visual feedback). But it's sufficient to
demonstrate the workflow and get feedback.
https://github.com/user-attachments/assets/ce2992e1-2794-43d6-baef-d10750d8d8e8
