feat: more efficient hydration markers

[Rich-Harris]

Currently, if and each blocks insert additional hydration markers to allow graceful recovery in the case of an SSR/CSR mismatch. These markers are removed from the DOM during hydration.

We can simplify things by encoding this information in the closing hydration marker. That way, no removal is necessary — we just inspect anchor.data. As a bonus, the SSR'd HTML is shorter.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: 9700ee5

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[dummdidumm]

What exactly is the encoding? The ! ? This should be commented somewhere else it's easy to overlook / harder to understand why it's added.

[Rich-Harris]

fair point — pushed a commit