_structuredClone dispatches platform-object hooks as hook(value, seen), but the
CryptoKey hook took only `src` and never touched the memo, and hook branches
return before the generic seen.set. So one CryptoKey referenced twice in a graph
cloned into two distinct objects (and re-registered its key material each time),
where structuredClone preserves reference identity within a graph.
Consult and populate `seen` in the hook. Adds a parity test asserting a key
reached twice clones to one shared object that crypto.subtle still accepts.
Andrey Mnatsakanov
8698afc065