fix: discard preload fork when navigating to a different route

[gordysc]

Fixes #14987

When preloading a route via data-sveltekit-preload-data="hover" and then navigating to a different route, the preload fork was not being discarded. This caused the orphan fork to interfere with subsequent DOM updates, resulting in the URL changing but page content remaining stale - particularly noticeable when navigating between routes with different layouts.

The issue was at line 1696 in client.js: when the navigation intent didn't match the preloaded route's ID, we correctly set load_cache_fork = null but only cleared the load_cache reference with load_cache = null. This failed to call fork.discard() on the orphan fork.

The fix conditionally calls discard_load_cache() (which properly discards the fork) when we're not using the preloaded fork, instead of just clearing the reference.

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[hmnd]

Much needed fix! There's also this related underlying issue too sveltejs/svelte#17335

[teemingc]

I don't think this PR fixes the issue. I tested it against the reproduction from the issue and it does not work. The tests in this PR also pass even without the fix. I'm going to close this to keep the PR queue clean but let me know if you want me to reopen it.