Deprecate DOMRemoteReceiver and RemoteReceiverElement

[MitchLillie]

TL;DR: Mark the DOM-backed receiver APIs as deprecated without removing or changing their behavior.

What changed:

• Added @deprecated annotations to DOMRemoteReceiver and its public policy/options types.
• Added @deprecated annotation to RemoteReceiverElement.
• Documented the recommended RemoteReceiver and framework-specific receiver alternatives.
• Added a @remote-dom/core changeset.

What this does not change:

• Existing exports and runtime behavior remain intact.
• RemoteReceiver, SignalRemoteReceiver, RemoteMutationObserver, RemoteRootElement, RemoteFragmentElement, compatibility adapters, and polyfill hooks are unchanged.
• BooleanOrString is intentionally unchanged.

Validation:

• Focused receiver and element tests: 7 files, 143 tests passed.
• RemoteReceiverElement tests: 5 tests passed.
• pnpm type-check
• pnpm lint