Layout breakpoints cannot be moved by a consuming product #2

Current state: RAIL_BREAKPOINT and DENSE_BREAKPOINT are module constants in a published brick, so a product with a wider rail cannot move the point where the navigation folds. DENSE_BREAKPOINT is exported but used nowhere inside the package, while base.css spells 639px by hand, so the two copies can drift with nothing pinning them together.

Proposal: expose them as custom properties or build options and pin the stylesheet to the same source.

Trigger: a product moves the fold point without forking, and one value drives both.