fix: write subscription updates to the subscribed key

[kwy404]

When several useSWRSubscription hooks share a key, the subscription is created once by the hook that mounts first, and its next callback writes data through that hook's swr.mutate. That bound mutate always targets the hook's current key (keyRef.current), not the key the subscription was created for. So if the first hook switches to another key while another hook keeps the shared subscription alive, every update for the old key is written into the cache of the new key. The hook that stayed on the old key stops updating, and the hook that moved shows data from the wrong source. Errors are not affected because next already writes them with a cache helper bound to subscriptionKey.

The fix writes data with internalMutate(cache, subscriptionKey, data, false), the same key the error path uses.

Test: should keep updating a shared key after one hook switches keys in test/use-swr-subscription.test.tsx. One component has two hooks on keyA, switches the first one to keyB, then emits on keyA. Before the fix it renders a2:a1 (the keyB hook got the keyA data and the keyA hook is stale) and the test fails. After the fix it renders undefined:a2 and the test passes. The other subscription tests, tsc, oxlint and oxfmt pass.