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This is an initial, partial implementation of a cleanup mechanism for the experimental Cache API. The idea is that consumers of the Cache API can register to be informed when a given Cache instance is no longer needed so that they can perform associated cleanup tasks to free resources stored in the cache. A canonical example would be cancelling pending network requests. An overview of the high-level changes: * Changes the `Cache` type from a Map of cache instances to be an object with the original Map of instances, a reference count (to count roughly "active references" to the cache instances - more below), and an AbortController. * Adds a new public API, `unstable_getCacheSignal(): AbortSignal`, which is callable during render. It returns an AbortSignal tied to the lifetime of the cache - developers can listen for the 'abort' event on the signal, which React now triggers when a given cache instance is no longer referenced. * Note that `AbortSignal` is a web standard that is supported by other platform APIs; for example a signal can be passed to `fetch()` to trigger cancellation of an HTTP request. * Implements the above - triggering the 'abort' event - by handling passive mount/unmount for HostRoot and CacheComponent fiber nodes. Cases handled: * Aborted transitions: we clean up a new cache created for an aborted transition * Suspense: we retain a fresh cache instance until a suspended tree resolves For follow-ups: * When a subsequent cache refresh is issued before a previous refresh completes, the refreshes are queued. Fresh cache instances for previous refreshes in the queue should be cleared, retaining only the most recent cache. I plan to address this in a follow-up PR. * If a refresh is cancelled, the fresh cache should be cleaned up.