style(design): FINDING-003 — every text step clears AA

The dim step measured 2.95:1 on the module plate (row state words, the composer
hint row, the staleness line) and the placeholder 2.53:1; light mode failed on the
composer hint (3.11:1) and the cube label (4.24:1). DESIGN.md section 21 requires
sufficient contrast including the dim values, so the token and the rule contradicted
each other.

Raise the label and dim steps in both themes and document the ladder in DESIGN.md.
Measured after: dark 4.76:1 to 7.02:1, light 4.55:1 to 5.85:1, four distinct steps
kept in each theme.