Implements the WHATWG structuredClone(value, { transfer }) global.

Architecture

Follows the post-context JS builtin pattern, same as the performance API in #430:

• Native stays thin. NativeScript/runtime/StructuredClone.{h,cpp} exposes a one-function binding bag (clone(value, transferArray)) and runs BuiltinId::kStructuredClone. The clone is a v8::ValueSerializer → v8::ValueDeserializer round-trip inside the one isolate, which is what StructuredDeserialize(StructuredSerializeWithTransfer(...)) reduces to when there is no second agent.
• All WebIDL coercion is portable JS. NativeScript/runtime/js/structured-clone.js owns the argument checks and the sequence<object> conversion for transfer. It contains nothing iOS-specific and is intended to be reused unchanged by the Android runtime.

ArrayBuffer membership in the transfer list is brand-checked from JS through the captured ArrayBuffer.prototype.byteLength getter (tamper-proof, and correctly excludes SharedArrayBuffer, which is not transferable); the native side re-checks with IsArrayBuffer().

Adds the SymbolIterator primordial.

V8 14.9 notes

• ArrayBuffer::Detach() on a non-detachable buffer aborts the process — it does not throw. The IsDetachable() pre-check before detaching is therefore load-bearing, not defensive. The deprecated void Detach() is avoided in favour of Detach(Local<Value>()).
• The SharedArrayBuffer/shared-value delegate hooks must be overridden to keep the DataCloneError name. With a delegate installed, V8's base GetSharedArrayBufferId and AdoptSharedValueConveyor throw a plain Error directly onto the isolate and never route through ThrowDataCloneError, so without these overrides a SharedArrayBuffer would surface an unnamed error.
• Release() must be called and its buffer free()d even after a failed write — the default delegate grows the buffer with realloc(), and the memory is owned by the caller after Release() regardless of whether serialization succeeded.
• Registering a buffer via TransferArrayBuffer makes the serializer skip its was_detached check, so detaching after a successful WriteValue (rather than before) is the correct ordering.

Deviations from the specification

• DataCloneError is an Error with .name = "DataCloneError", not a DOMException. This runtime has no DOMException; the shape matches how native exceptions are already surfaced (see docs/error-handling.md).
• ArrayBuffer is the only transferable type. No MessagePort, ImageBitmap or ReadableStream exists here, so any other value in the transfer list is a DataCloneError.
• SharedArrayBuffer shares (a new SAB over the same backing store) and is not transferable — a SAB in the transfer list is a DataCloneError.
• Host objects are never cloneable. A native/interop wrapper serialized without its native counterpart would deserialize into a wrapper around nothing, so WriteHostObject reports it as uncloneable.

Documented in docs/structured-clone.md.

Tests

Shared cross-runtime suite: NativeScript/common-runtime-tests-app#26 (54 specs — clone semantics, graph identity and cycles, transfer, SharedArrayBuffer sharing, worker message transfer, DataCloneError cases). It runs from runAllTests() and gates itself on structuredClone being present — a runtime without the API reports one visible pending spec instead of failures, so the tests PR is safe on Android's master immediately. An unguarded canary in RuntimeImplementedAPIs.js asserts the global exists on iOS, so the gate cannot hide a regression here.

Full TestRunner suite green on the simulator: 1010 tests, 0 failures, 11 pre-existing skips.

Merge sequencing

Done — the tests PR (NativeScript/common-runtime-tests-app#26) is merged and the submodule pointer here references the resulting master (which also carries the self-gating Performance suite from #25; it skips on this branch as one pending spec, verified in the full run). No remaining ordering constraints.

Independent of #430 — both branch off main. Trivial conflicts are expected in whichever merges second: the Runtime.mm post-context init block, project.pbxproj, tools/js2c-inputs.xcfilelist, js/primordials.js, and (between the two tests PRs) shared/index.js and RuntimeImplementedAPIs.js.

Serialization consolidation (added scope)

Follow-up review moved the shared machinery into one core so structuredClone and worker postMessage can never diverge:

• StructuredSerialization.{h,cpp} (tns::serialization) now owns the serializer/deserializer delegate pair, DataCloneError construction, transfer-list validation (duplicates, detached/non-detachable — the pre-check is load-bearing since Detach() aborts on 14.9), and the register→write→claim→detach ordering. SerializedValue keeps serialize/deserialize as separate halves because a worker message deserializes on a different isolate. Message.cpp is deleted (Message.hpp is a one-line alias), StructuredClone.cpp is a thin same-isolate consumer, and the dead Node message-port scaffolding plus the legacy JSON Worker::Serialize path are pruned.
• postMessage gains the ArrayBuffer transfer list on both entry points (Array of ArrayBuffers, TypeError otherwise), using the shared validation. A latent bug fixed on the way: postMessage previously ignored Serialize failures and posted an unwritten buffer.
• Host objects stay intentionally asymmetric, encoded in one place: HostObjectPolicy { kReject, kDegrade }. structuredClone rejects with DataCloneError (spec); postMessage keeps the shipped degrade-to-{} behavior that the 2016 cross-runtime worker test asserts on both runtimes (the write/read sides are a matched zero-byte pair over V8's own kHostObject tag — balanced, not corrupt). Unifying on reject is a coordinated follow-up with the Android runtime; it will be a one-line policy change here.
• SharedArrayBuffer now shares through structuredClone (new SAB over the same backing store, both directions asserted by the suite) instead of throwing — that deviation is removed. Errors from both entry points are unified on NativeScriptException with the DataCloneError name, so worker-path errors keep fullMessage and structuredClone errors gain it.

Summary by CodeRabbit

• New Features

  • Added global structuredClone() support for cloning values, circular references, and transferable ArrayBuffer objects.
  • Added validation and errors for unsupported values, invalid transfer lists, and host objects.
  • Added transfer-list support to worker postMessage() calls.

• Documentation

  • Documented cloning behavior, supported values, transfers, errors, and worker messaging.

• Tests

  • Added coverage confirming structuredClone() is available at runtime.