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