v4.21.3

Focus: Dark-theme polish — no more white flash on server-rendered code blocks.

Changes

• 🎨 Fixed dark-theme code blocks painting a light first frame: the tone classes hydrate as the serialized white and only flip to dark post-mount, so the light/dark palettes are now keyed on the body theme too — white rules apply only under body:not(.body--dark) and the dark surface/token palette also applies under body.body--dark, making the server-rendered markup born dark and the class flip a visual no-op
• ✅ Verified with a serialized-markup fixture (frame, gutter and Prism token colors all compute the dark palette pre-hydration) and screenshots in both themes; 335 tests passing