feat(server): add an earlyHints option for 103 Early Hints

[lxsmnsyc]

Closes #1935.

Adds an opt-in earlyHints option. With it on, SolidStart sends the page's assets as 103 Early Hints, so the browser can start downloading them while the server is still rendering.

// vite.config.ts
solidStart({ earlyHints: true });

Change

• New server/early-hints.ts. sendEarlyHints(event, assets) converts the page's <link> assets to Link values and passes them to h3's writeEarlyHints. On Node, that writes a real 103 response. On other runtimes, h3 adds the same values as Link headers on the final response, which is the fallback the issue asked for.
  • Stylesheets become rel=preload; as=style and scripts rel=modulepreload, matching the page's own <link> tags.
  • URLs are passed through encodeURI, since header values must be ASCII and a chunk name can contain any character a route file name can.
  • It does nothing outside production, with the option off, or once the response headers are sent.
• Entry assets (client entry, app entry, merged CSS) are sent from createPageEvent, before rendering starts.
• Route assets are sent from the server-side lazy() wrapper in shared/lazy.ts, in the component it returns. That code runs on every render. My first prototype hooked the module-level part, which lazy() caches, so route hints only went out on the first request after the server started.
• Off by default, like Nuxt's experimental.writeEarlyHints, because some proxies and clients mishandle 1xx responses.

Testing

Production builds of the fixtures, with a raw Node HTTP client that logs each 1xx response (information event) and the final one, in ms from the request:

Setup	Responses for /about (with a route CSS file added)
basic, Node, earlyHints: true, stream mode	103 entry CSS + 3 entry JS (7ms), 103 about.css + about.js (11ms), 200 (15ms)
basic, Node, earlyHints: true, mode: "async"	both 103s, then 200
basic, Node, option off	200 only
basic, Bun (NITRO_PRESET=bun), earlyHints: true	no 103; 200 with a Link header holding all 6 values (entry and route)
experiments, Node, /漢字	103 with the entry assets, no errors

• In stream mode the route hint still arrives before the final headers. Once a route module is loaded, its component renders synchronously, before the handler returns the stream.
• The hints match each page's own <link> tags. For /漢字, the page itself has no route-level <link> for its chunk (漢字-….js exists in the build), so no route hint is sent either. That looks like an existing issue with non-ASCII route ids in the asset lookup, separate from this PR.
• The client bundle contains no hint code.
• New early-hints.spec.ts (6 tests): the Link values, including encoding a non-ASCII URL, and skipping <style> and non-preload links. Also, that hints are sent in production with the option on, and that nothing is sent with the option off, outside production, after the headers are sent, or with nothing to hint.
• @solidjs/start test:ci (246 tests), typecheck, validate-imports and build pass. apps/tests unit tests (6), Playwright (42), bundled-dev Playwright (42) and e2e:base (3) pass.

Expected benefit

On pages that stream quickly, the 103s arrive only milliseconds before the shell, whose <link rel="modulepreload"> tags the browser would find almost as soon. The gain is on pages that take a while before the response, such as mode: "async", deferStream, or slow middleware: there the browser can fetch the assets while the server is still working.

A matching PR for the 1.x branch follows.

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Latest commit: 1cf2ab2

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