design: gradient text, purple-blue gradient and violet hex rules #87

Goal

Ship the first three design rules, scored in the same PR, so the family has a measured baseline. This PR closes the corpus and style-layer issues too.

Candidate tells

• Gradient text (SLOP052): a block with background-clip: text plus a *-gradient( value; or one class list with bg-clip-text plus bg-gradient-to-*, bg-linear-* or bg-radial*. Fix: use a solid text colour.
• Purple-blue gradient (SLOP053): a gradient with at least 2 colour stops, every stop at saturation >= 0.4 and hue 225 to 330 degrees, at least one stop at hue 255 to 290; or Tailwind from- with purple, violet or indigo plus a via- or to- with purple, violet, indigo, blue, cyan, pink or fuchsia. Fix: use the brand's own colours.
• Hand-copied violet hex (SLOP054): 7c3aed, 8b5cf6, a855f7, 9333ea, 7e22ce, 6d28d9, 6366f1, 764ba2 or 667eea outside gradient values; skips --tw-* properties. Fix: use a design token.

Disjointness: the gradient-text rule owns the clip span, the purple-blue rule owns the gradient, and the hex rule skips gradient values.

Acceptance criteria

• Each rule has a fixture with expect: markers and a clean twin carrying none, for every language it declares.
• just corpus-score rows for each rule are pasted into the PR. Rows that separate the human and AI splits ship at Tier B default-on; rows that do not stay Tier C, default-off, with the measurement in the PR body.
• Panel disjointness: grep src/ for every span and phrase against SLOP009, SLOP018, SLOP021, SLOP044 and the prose buzzword panels; drop what another rule owns.
• README rule table and CHANGELOG.md updated in the same PR.
• just check is green, and the installed binary produces zero new findings on this repo.

Blocked by

Blocked by #85, #86

Part of #98