Forward focus and scrollIntoView to the host

[airhorns]

What changed

focus() and scrollIntoView() are missing from polyfilled elements, even though Remote DOM already has a method-call channel and both operations can be performed by the rendered host element. Remote code type-checked against lib.dom therefore fails at runtime before it can use that channel.

This adds a general imperative-method hook to @remote-dom/polyfill, implements focus() and scrollIntoView() on the polyfill's shared Element abstraction, and wires the core polyfill hook to the connected RemoteConnection. Options are forwarded unchanged. Standalone or disconnected polyfilled elements keep browser-like no-op behavior when no host hook is available.

The core regression test connects a real RemoteRootElement and RemoteReceiver, registers the host implementation, and verifies both method calls end to end.

These methods live on the shared Element class because that is the current concrete class returned for ordinary and custom HTML elements on main. #626 separately proposes concrete HTML constructor identity.

getBoundingClientRect() is deliberately not included: it is synchronous and returns host layout data, which cannot be faithfully implemented over an asynchronous remote connection.

Validation

• pnpm exec vitest run (178 tests)
• pnpm lint
• pnpm type-check
• mise exec node@20.20.0 -- pnpm --filter @remote-dom/polyfill --filter @remote-dom/core build

[developit]

Suggested change

	if (arg === undefined) {
	this.callHostMethod('scrollIntoView');
	} else {
	this.callHostMethod('scrollIntoView', arg);
	}
	this[HOOKS].callMethod?.(this as any, 'scrollIntoView', arg);

[developit]

Suggested change

	if (options === undefined) {
	this.callHostMethod('focus');
	} else {
	this.callHostMethod('focus', options);
	}
	this[HOOKS].callMethod?.(this as any, 'focus', arg);

[developit]

This makes the method callable from userland. Callsites must just use this[HOOKS].callMethod?.(this as any, 'scrollIntoView', arg) directly (see other suggestion comments).

Suggested change

	protected callHostMethod(method: string, ...args: unknown[]) {
	this[HOOKS].callMethod?.(this as any, method, ...args);
	}