Airframe theme-v15

The docs nav's active section is one shape, the ApprovalCard preview agrees with its README, and the brand book is
split by question (KAN-162). No token is added, renamed or removed, and no token value changes.

Docs-site mapping (airframe-docs.css):

• A section with its own index page (navigation.indexes) renders its link and its expand toggle as two active links.
  Each drew its own selected pill and product marker, so the active item split in two. The .md-nav__container
  now carries the one pill and the one 2px --af-product marker, and the two parts inside it are clear. Forced
  colors get one Highlight edge.

Components and guides (not in theme/; they ship with the registry and the skill):

• ApprovalCard: the deny reason field is neutral, with a hint, until Deny is pressed with it empty; the README shows
  the invalid state separately. The narrow card's ID is the tail number HGR-APR-8E7F, and its buttons read
  "Approve for this turn" and "Approve", not Allow.
• guide/brand.md (3,373 words) is now a hub with the principles, and nine pages, one per question: Writing, IDs,
  Color, People, Type, Layout, Motion, Iconography and Consume. The old section anchors still resolve on the hub.