build(frontend)!: knip drops --production; plain knip stays

The production pass (7ed2ac4e, E32) asked "is this export used by what
ships?". No real hit: its only findings were 37 exports a test imports,
answered with 36 `@internal` JSDoc tags (E34 S2) — test-only exports, not
defects. Since then it has only ever asked for another tag. The tags go
with it (36, no longer read by anything), and so do the production-only
`!…!` exclusions in knip.jsonc and their register rows. Plain knip (unused
files, exports and dependencies) keeps running in the gate.

frontend/src/lib/colorMath.ts

@@ -34,7 +34,6 @@ export function hueGap(a: string, b: string): number {
 
 /** `color-mix(in srgb, a w, b)` for two opaque hex colours: the channels mix gamma-encoded, as the
  *  browser does in the srgb space. */
-/** @internal exported for theme.contract.test.ts */
 export function mixHex(a: string, b: string, w: number): string {
   const [x, y] = [rgb(a), rgb(b)];
   return `#${x