Add is() and isNot() element identity assertions

[NullVoxPopuli-ai-agent]

Implements #2214.

Adds two new assertions for comparing the target element against another element by reference, while using qunit-dom's element serialization for readable failure messages:

assert.dom('#title').is(element);
assert.dom(el).isNot(otherEl);

Previously the only way to assert that two elements are the same node was assert.strictEqual(el, otherEl), which produces useless failure output for DOM nodes, or piecewise hasAttribute()/hasClass()/hasText() assertions that can't establish identity at all.

Details:

• is(expected) / isNot(unexpected) compare by reference (===); the failure message serializes both elements (e.g. Element #title is the same element as h2#subtitle with actual: h1#title, expected: h2#subtitle).
• When two different elements serialize identically, is() annotates the actual value with (a different element with the same description) so the failure isn't a confusing x !== x diff.
• Both throw a TypeError when passed a non-Element, matching the existing conventions for JavaScript users.
• Unit tests cover selector and element targets, missing targets, identical-serialization failures, invalid arguments, and chaining. API.md regenerated with pnpm docs (the link-reference renumbering accounts for most of the diff there).

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