Add opt-in structuredClone

[lahma]

Part of the opt-in web API series (#3079): structuredClone(value, { transfer }) behind WebApiFeatures.StructuredClone (in Default). Usual rules: BCL only, net8+, opt-in, default engine unchanged.

Implements HTML's StructuredSerializeWithTransfer/StructuredDeserializeWithTransfer (https://html.spec.whatwg.org/multipage/structured-data.html) fused into a single walk — structuredClone has one realm, so the memory map keys on source-object identity, and the fusion is argued unobservable in the class doc (identical traversal order → identical getter order; a mid-walk throw discards an unreachable clone; transfer still runs strictly after the walk).

Covered: primitives; boxed Boolean/Number/String/BigInt; Date; RegExp (source+flags, lastIndex starts 0); ArrayBuffer incl. resizability, detached → DataCloneError; TypedArrays/DataView (two views over one buffer deserialize as two views over one cloned buffer — the memory map gives it for free); Map/Set; Array with holes preserved; Errors per the spec table (plus stack carried, as both HTML and WebIDL say a UA "should"); DOMException (it is WebIDL-[Serializable]); plain objects via own enumerable string keys read with Get (getters run, per spec); circular references. Symbols, functions, proxies and host-wrapped objects throw DataCloneError — the recognition is deliberately positive (only object shapes Jint can prove ordinary clone), so nothing ever silently loses data. transfer accepts ArrayBuffers, with the spec's duplicate/detached/non-transferable error order pinned.

The walk is iterative, not recursive — there is no depth limit at all. The motivation is empirical: the same 100k-deep {next:{next:…}} graph run through the engine's recursive JSON.stringify kills the process with an uncatchable 0xC00000FD stack overflow; structuredClone of that graph (and equally deep array and Map chains) completes in milliseconds, and progress is charged against the engine's execution constraints so a huge clone stays interruptible by timeout/cancellation.

94 tests including the deep-graph cases, transfer error ordering, realm-correct prototypes, and the public-surface pins. Verified: all-TFM build, both suites × both frameworks × both host-contract-verification configurations.

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