chore: build streamed responses from async generators

[Nic-Polumeyv]

Four places hand-write the producer side of a stream. server/page/render.js and server/data/index.js each construct a ReadableStream whose start enqueues an encoded head, loops an async iterable of chunks and closes; form-utils.js LazyFile.stream() drives read_range through a pull adapter that re-tracks a cursor the generator already knows; and utils/streaming.js create_async_iterator implements [Symbol.asyncIterator]/next by hand with two index cursors.

This replaces them with async generators and ReadableStream.from, which server/index.js already relies on for promised read results. The two response pumps share a stream_text(head, chunks) helper, LazyFile.stream() becomes a for await over read_range, and iterate becomes an async function* over the deferred list.

Two deliberate deltas: the response streams lose type: 'bytes' (added alongside the encoding fix in #9136; Response only requires Uint8Array chunks, which .from over encoded strings provides), and an incomplete LazyFile now errors with an Error instead of a bare string.

---

Please don't delete this checklist! 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
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

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

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 5ebbd02:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/5ebbd028f682ba700531fcf2da0e2c127b4bd5f9

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16847

[changeset-bot]

🦋 Changeset detected

Latest commit: 5ebbd02

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

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	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

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-16847-svelte.vercel.app/

[Nic-Polumeyv]

Pushed a fallback for runtimes without ReadableStream.from. It's still unreleased in Bun (oven-sh/bun#33193 landed on main, latest release is 1.3.14), so streamed responses would throw there. Also routes the #16622 site through it, and fixes the lint failure.