fix: preserve canonical metadata after Notion hydration - #195

Thanks for working on #193 and for the detailed test.

We can't merge this one, for two reasons:

1. Notion client overwrites canonical URL and og:url in the browser #193 was already fixed by Keep the canonical URL and og:url after the Notion client loads #194, which was merged before this PR was reviewed. The client script now keeps link[rel=canonical], og:url and twitter:url on the worker's canonical URL.
2. getCanonicalUrl() references MY_DOMAIN inside the client script, where it isn't defined; the worker only interpolates it as ${MY_DOMAIN}. The top-level applyMeta() call then throws ReferenceError: MY_DOMAIN is not defined, and the rest of the client script never runs. In wrangler dev with our render-check page, pretty URLs were lost (/subpage became /Render-Check-Subpage-<id>), and the error was thrown hundreds of times per page load. The new test in this PR fails with the same error (npm test: 25 passed, 1 failed).

Running the generated client script in node:vm with a small DOM stub is a nice idea, since the client script has no unit tests today. If you'd like to contribute that as a separate PR against the current master, testing the behavior from #194, we'd be happy to review it.

Closing this as superseded by #194.