Next.js 16.3+ Cache, PPR, and Server Actions Bugs Causing Hydration Errors, Stale Data, and 500 Errors

Multiple production issues identified in Next.js versions 16.3+ regarding Partial Prerendering (PPR), "use cache", Server Actions, and router.refresh(). These bugs cause 500 errors on server action requests when shells are stale, React #418 hydration errors when using cache in layouts with partial prefetching, aborted background cache regenerations due to unmanaged waitUntil promises, and silently failing router.refresh() calls on streamed pages.

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