docs(claude): refresh PRODUCT.md and DESIGN.md from the implemented UI

- Rewrite DESIGN.md from source: add Layout and Shapes, restore the
  spec's canonical headings so the file parses, and document the
  status toast, header elevation, brand lockup, environment badge,
  splash shadow vocabulary, and outlined-field convention
- Correct values against source, including the splash card lift and
  text halos, which are Aggie Blue tinted rather than black
- Record product context in the current schema: Positioning, Operating
  Context, Evidence on Hand, and Product Principles, dropping the
  deprecated Register section
- Split the two files by scope so neither restates the other, which
  retires PRODUCT.md's duplicated anti-reference and accessibility
  lists in favor of the DESIGN.md sections that already own them
- Flag the drift found while scanning: two darkened golds, three
  untokenized literals, and two values shipping under blue-70

Author: rlorenzo