Next.js 16 Caching, PPR, and Server Actions Bugs causing Hydration Mismatch, Stale Data, and Failures

Multiple severe caching, Partial Prerendering (PPR), "use cache", and server action bugs in Next.js 16 cause issues like hydration mismatch (#418), silent router.refresh failures on streamed pages, server action POST errors on stale shells, background regeneration cutoffs on Vercel, and incorrect cache expiration bails.

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