theme: a document-cache entry replays the first visitor's initialTheme to every visitor #978

Problem

The document cache (src/cache/document-cache.ts, opt-in via s-maxage) stores the full HTML response, and that includes metadata.initialTheme. Without a PPR capture, payloadInitialTheme reads it from the current visitor's theme cookie (full-payload.ts:100). Every visitor served that cached document therefore gets the first visitor's initialTheme. A visitor with no stored theme keeps the other visitor's useTheme().theme / resolvedTheme for the page's lifetime: ThemeProvider re-syncs only from an explicit cookie or localStorage value (ThemeProvider.tsx ~153).

The <html> class is correct, because the inline theme script reads document.cookie before paint. So the symptom is useTheme() output, such as a toggle label or icon, showing another visitor's theme.

This is the same mismatch the #971 fix (PR pending) closes for PPR shells, where the capture now stores themeConfig.defaultTheme. This report comes from reading the code; no test has run it yet.

Proposal

When a document is rendered for the document cache, set initialTheme to the configured default theme and ignore the visitor's cookie, the same way the shell capture does. Add a test with a dark-cookie visitor first, then a no-cookie visitor.