Fix Node#contains infinite loop for indirect descendants

[yzxcj797]

What?

Node#contains() never terminates for any indirect descendant: the walk re-reads the original node's parentNode on every iteration instead of advancing.

contains(node: Node | null) {
  let currentNode: Node | null = node;
  while (true) {
    if (currentNode == null) return false;
    if (currentNode === this) return true;
    currentNode = node!.parentNode;   // ← never advances past the argument's own parent
  }
}

For a direct child the first step happens to hit the receiver and returns true — which makes the bug look intermittent: only deeper nesting hangs. For an indirect descendant the loop re-reads the same middle node forever. In a synchronous environment — a Web Worker running the polyfill as its DOM — that single call deadlocks the whole event loop with no exception and no recovery short of terminating the worker.

This is reported downstream in the wild: twenty's front-component Remote DOM worker freezes permanently whenever a component hit-tests with Element.contains() on anything nested deeper than one level (twentyhq/twenty#24573); a maintainer-confirmed MEDIUM with the worker's timers and direct fetch heartbeats all stopping at the call site.

Fix

Advance the walk with currentNode.parentNode, per the Node.prototype.contains contract.

Testing

New packages/polyfill/source/tests/node.test.ts (vitest, same harness as the existing serialization.test.ts): direct child, indirect descendant (the hanging case), self, detached node, sibling subtrees, and null — 6/6 pass.

Differential: on the previous code the indirect-descendant case does not merely fail — it hangs the runner indefinitely (the run had to be killed after several minutes; an isolated child-process repro of the exact loop also never terminates against a three-level chain). With the change the whole file passes in under five seconds.