hydrogen NavBar: current-page state that actually works, and is cached correctly

[romain-pm]

Part of #749. Split out of #751 because it changes what the reference sample renders, rather than adopting an API mechanically.

samples/hydrogen's NavBar decides the current page with page === mainNode — reference equality on two Graal host proxies, which are not the same object. So aria-current never appears on the canonical navigation sample today. <JLink node={page} /> compares identifiers, and it starts appearing.

That is a fix, but it is newly visible output on the sample everyone copies, and it comes with a caching obligation the monorepo has never mentioned.

• Move both link sites to <JLink node={page} />, label derived from the page.
• Declare properties: { "cache.mainResource": "true" }. aria-current is a function of the main resource, and PathCacheKeyPartGenerator leaves the main resource out of the fragment cache key unless the view opts in — so a nav shared across pages is cached once and replayed with the wrong entry marked current. See cache.mainResource: make current-page state safe to cache #755.
• Confirm on a running instance that the nav is not over-cached: render it on two pages and assert aria-current follows.
• Decide whether the getting-started nav chapter needs the same treatment.

Depends on #751 (the <JLink> implementation).