[compiler] `validate` rejects document-shell templates (`<html><head><body>`) — fatal in client compiles since rc.5 made validate failures errors

## Summary

The `validate` pass round-trips each DOM template through a body-context fragment parse and
compares serializations. A template whose root is `<html>` (a root document: `<html><head>…
</head><body>…</body></html>`) can never pass that comparison — body-context `innerHTML`
parsing strips the `<html>`/`<head>`/`<body>` wrappers, so the "browser" serialization never
matches the input, regardless of whether the markup is actually well-formed.

Through 2.0.0-rc.4 this printed the malformed-HTML warning and emitted anyway. Since 2.0.0-rc.5
(#3099 — the right call for genuine restructuring) it is a compile error, so a module containing
root-document JSX fails to compile at all in `generate: 'dom', hydratable: false` mode. Merely
importing the module is fatal; no template has to be instantiated.

The validator already handles context-sensitive fragments: `<td>`/`<tr>`/`<col>`/table components
are wrapped in a synthetic `<table>` before parsing. `^<html>` needs the analogous treatment
(validate in the document context, or skip/downgrade for document shells).

Where this bites: apps whose root component owns the document shell — e.g. a TanStack Solid Start
root route — when that component is compiled in plain client mode, which is the natural
configuration for a jsdom/vitest component-test project. The route tree transitively imports the
root route, so the whole test import chain dies. (Dev/build pipelines using hydratable client
compiles are unaffected, which hides the problem until someone runs component tests.)

Two smaller observations while here:

- The thrown error appends `(This is an error on an internal node. Probably an internal error.)`
  instead of a source code frame, so this configuration still loses the user-code location
  promised by #3099.
- `hydratable: true` client compiles do not hit the error for the same input, so the mode that
  actually hydrates real documents is already outside the check's reach.

## Reproduction

```js
// repro.mjs — pnpm add @babel/core @babel/plugin-syntax-jsx @solidjs/babel-plugin@2.0.0-rc.6
import { transformAsync } from '@babel/core';
import solid from '@solidjs/babel-plugin';
import jsx from '@babel/plugin-syntax-jsx';

const src = `
export function Root(props) {
	return (
		<html>
			<head><title>App</title></head>
			<body><div id="app">{props.children}</div></body>
		</html>
	);
}
`;

for (const opts of [
	{ generate: 'dom', hydratable: false }, // THROWS — the bug
	{ generate: 'dom', hydratable: true }, // OK
	{ generate: 'ssr', hydratable: true }, // OK
]) {
	try {
		await transformAsync(src, {
			filename: 'root.tsx',
			plugins: [jsx, [solid, { moduleName: '@solidjs/web', ...opts }]],
		});
		console.log(JSON.stringify(opts), '→ OK');
	} catch (e) {
		console.log(JSON.stringify(opts), '→ THROWS:', e.message.split('\n')[0]);
	}
}
```

Save as `repro.mjs` and run `node repro.mjs`. Tested on Linux with Node 24.19.0; this is a
compile-time issue, so no browser is involved.

Output at 2.0.0-rc.6:

```
{"generate":"dom","hydratable":false} → THROWS: /path/to/root.tsx: The HTML provided is malformed and will yield unexpected output when evaluated by a browser.
{"generate":"dom","hydratable":true} → OK
{"generate":"ssr","hydratable":true} → OK
```

At 2.0.0-rc.3/rc.4 the same input prints the diagnostic as a warning and compiles.

## Expected behavior

A well-formed document-shell template validates (parsed in the document context, the way table
fragments are parsed in a synthetic table), or at minimum the document-shell case degrades to the
pre-#3099 warning instead of failing the compile.



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[compiler] `validate` rejects document-shell templates (`<html><head><body>`) — fatal in client compiles since rc.5 made validate failures errors #3259

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[compiler] validate rejects document-shell templates (<html><head><body>) — fatal in client compiles since rc.5 made validate failures errors #3259

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[compiler] `validate` rejects document-shell templates (`<html><head><body>`) — fatal in client compiles since rc.5 made validate failures errors #3259