Preserve __proto__ keys in structuredClone

[OskarEichler]

Summary:

The structured-clone polyfill writes enumerable keys through assignment, so an own __proto__ data property mutates the clone's prototype and disappears. Route property writes through one helper that retains ordinary assignment on the common path and uses defineProperty only for this exceptional key across array, fast-object, and slow-object traversal.

Fixes #58216.

Changelog:

[GENERAL] [FIXED] - Preserve own proto data properties and normal prototypes in structuredClone.

Test Plan:

• Exact Hermes baseline produced no own descriptors and replaced both Object/Array clone prototypes with the property value.
• Fixed focused Fantom suite: 33/33 passed; own descriptors, ordinary prototypes, and deeply cloned values match native structuredClone.
• Fresh yarn flow-check: 0 errors.
• Targeted ESLint, Prettier, and git diff --check passed.

No UI change; screenshots are not applicable.