fix: load preparser.ts from theme roots

[kaigritun]

Fixes #2402

Problem

The setup/preparser.ts file in themes was being ignored while other setup files like shiki.ts were correctly loaded.

Root Cause

In packages/slidev/node/setups/preparser.ts, the roots array only included:

• Addon roots
• User root

But NOT theme roots. Compare with how setupShiki() receives resolved.roots which includes themes.

Solution

Resolve the theme root from headmatter (using the same logic as options.ts) and include it in the roots array passed to loadSetups().

Testing

• All 101 existing tests pass
• ESLint passes

Changes

import { resolveAddons } from '../integrations/addons'
+import { resolveTheme } from '../integrations/themes'
import { getRoots } from '../resolver'
import { loadSetups } from './load'

export default function setupPreparser() {
  injectPreparserExtensionLoader(async (headmatter, filepath, mode) => {
    const addons = Array.isArray(headmatter?.addons) ? headmatter.addons : []

+   // Resolve theme from headmatter (matching logic from options.ts)
+   let themeRaw = headmatter?.theme as string | null | undefined
+   themeRaw = themeRaw === null ? 'none' : (themeRaw || 'default')

    const { userRoot } = await getRoots()
+   const [, themeRoot] = await resolveTheme(themeRaw, filepath)
+   const themeRoots = themeRoot ? [themeRoot] : []

    const roots = uniq([
+     ...themeRoots,
      ...await resolveAddons(addons),
      userRoot,
    ])

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[antfu]

Why do you want to use a theme to change the preparser but not an addon? I believe this was on purpose to differentiate two integrations with different scope

[kaigritun]

Good question! I based this on consistency with existing behavior - themes already support setup/shiki.ts and other setup files via resolved.roots (which includes themes), but preparser was the exception.

The original issue (#2402) reports that users expect parity: if setup/shiki.ts works in themes, setup/preparser.ts should too.

That said, if the intent is for preparser customization to be addon-only, I can close this PR - just wanted to match the pattern established by other setup files. Happy to follow whichever direction you prefer!

[antfu]

Shiki is related to the theming, which is why it's included. We could update the docs to clear the behaviour instead.