Page shell invariants

Four things must be true of every page, on every route, at every breakpoint. They are invariants, not preferences: if one is false, the page is broken, even if it looks fine on your machine.

1. Margins are consistent and come from tokens.
2. The header stacks above the hero. Always.
3. Page height is stable. Nothing jumps.
4. Navigation prefetches on hover or focus.

1. Margins and gutters

One horizontal rhythm for the whole page. Sections do not each invent their own.