fix: observe existing simulator root for size updates

[ruben-prismic]

Description

Fixes slice zone size updates when .slice-simulator--root already exists before SimulatorManager initializes its listeners.

The existing MutationObserver only handles roots added later. This calls the existing observer setup once during listener initialization and adds a regression test for that behavior.

Checklist

• If my changes require tests, I added them.
• If my changes affect backward compatibility, it has been discussed.
• If my changes require an update to the CONTRIBUTING.md guide, I updated it.

How to QA

• Run npm run test.

---

^{Need help on this PR? Tag @codesmith with what you need. Autofix is disabled.}

---

Note

Low Risk
Small listener-init fix plus a unit test; no auth, data, or API contract changes.

Overview
Fixes slice zone size reporting when .slice-simulator--root is already in the DOM before listeners start. A MutationObserver only ran observeSimulatorRoot when children were added later, so an existing root never got a ResizeObserver until the DOM changed again.

SimulatorManager now calls observeSimulatorRoot() once right after wiring the mutation observer, reusing the same hook that attaches the resize observer. A regression test stubs ResizeObserver and asserts the existing root is observed when sliceZoneSizeAPI is enabled.

^{Reviewed by Cursor Bugbot for commit 727b2a1. Bugbot is set up for automated code reviews on this repo. Configure here.}