feat: add link prefetching support

[bartlomieju]

Summary

• Adds f-prefetch attribute for prefetching partial navigations ahead of time
• Supports four strategies: hover (default), viewport, load, none
• Container-level defaults via f-prefetch on parent elements (e.g. f-client-nav containers)
• Integrates with the existing partials system — prefetches the partial HTML, not full pages
• Respects navigator.connection.saveData and slow connection types
• Deduplicates in-flight requests and caches responses with 30s TTL
• Uses MutationObserver to handle dynamically added links

Usage

<nav f-client-nav f-prefetch="hover">
  <!-- All links prefetch on hover by default -->
  <a href="/about">About</a>
  <a href="/dashboard" f-prefetch="viewport">Dashboard</a>
  <a href="/heavy-page" f-prefetch="none">Heavy Page</a>
</nav>

Test plan

• All existing 120 tests pass
• Manual testing with f-prefetch attribute on links
• Verify hover strategy triggers fetch on pointerenter
• Verify viewport strategy triggers fetch when link scrolls into view
• Verify load strategy triggers fetch immediately
• Verify none strategy prevents prefetching
• Verify container-level defaults propagate to child links
• Verify Save-Data is respected
• Verify cache TTL expiry works correctly
• Verify cached response is consumed on navigation (no double fetch)

Closes #3788

[lunadogbot]

Strategy reads well — closest() for container defaults, IntersectionObserver for viewport, MutationObserver for dynamic links, navigator.connection.saveData + slow-type opt-out, single-shot cache consumption on click.

One blocker:

1. In-flight race in getCachedResponse: the function is sync and only checks prefetchCache. A user who clicks on a hover-prefetched link before its fetch resolves hits the inflightRequests window — prefetchCache.get(url) returns undefined, so partials.ts:fetchPartials falls through and issues a second fetch(partialUrl, init). Two concurrent requests for the same URL is the exact case the inflight map exists to prevent. The fix is to await the inflight promise:

   export async function getCachedResponse(partialUrl: URL): Promise<Response | null> {
     const url = partialUrl.href;
     const inflight = inflightRequests.get(url);
     if (inflight) await inflight;
     // ...existing TTL check + delete-and-return
   }

   The call site at partials.ts:363 already lives inside an async function, so await getCachedResponse(partialUrl) is a one-character change.

• nit: no tests for the new 239-line module. The eligibility rules (origin, target, fragment-only hrefs), strategy resolution (element vs. container vs. nested overrides), TTL expiry, and single-use consumption are all unit-testable with jsdom/a stubbed IntersectionObserver.
• nit: prefetchedUrls.add followed by prefetchCache.has(url) check at prefetch.ts:79-82 is unreachable in practice — anything in prefetchCache was added by a prior prefetch() call that also added to prefetchedUrls, so the earlier prefetchedUrls.has(url) guard already returned. The TTL-staleness branch you intended to hit will never fire.
• nit: `getStrategy` returns `"none"` when neither the element nor any ancestor has `f-prefetch`. With the current setup, that means prefetch is off by default unless opt-in — worth a brief doc note since the PR description shows it as default-hover within an `f-prefetch="hover"` container.