Flags as Code in Next.js

Introduces the new Flags SDK for Next.js and SvelteKit, enabling server-side evaluation of feature flags. Details guiding principles: flags as functions, server-side evaluation only, and flags accepting no arguments on the call side. Explains primitive flags and progressions with tradeoffs: hardcoded flags (slow to change, affects everyone) vs. overriding from Vercel Toolbar (session-specific overrides for testing). Discusses using environment variables and Edge Config as flag sources, and mentions OpenFeature integration. Highlights the benefit of precomputing feature flags for static page variants.