Replay view transitions on browser Back/Forward

[gkurt]

What

Today a view transition only plays when a navigation goes through the router — <Link viewTransition> or navigate({ viewTransition }). When the user presses the browser's Back/Forward buttons, the same navigation plays a hard cut instead of the transition.

I'd like an opt-in that replays the transition an entry was created with whenever the user traverses to it, so the animation is symmetric: a transition opted into on A → B plays on both B → A (back) and a later A → B (forward).

Why

• The asymmetry is visible. An app can animate A → B via a Link, then hard-cut when the user hits Back to return. Users notice the inconsistency.
• defaultViewTransition is all-or-nothing. The only way to animate traversals today is to turn on transitions for every navigation. There's no way to opt one flow in and have it round-trip.
• It matches intent. If you asked for a transition going one way, getting it on the way back is almost always what you wanted.

Open questions

• API shape — a dedicated opt-in option, or fold this into defaultViewTransition?
• Default — should replay be off by default (conservative), or on for navigations that already opted into a transition (treating the asymmetry as a bug)?

Prototype

Implementation, tests, and a runnable example are in #7697. Feedback on the direction and the two questions above would be very welcome before it moves forward.