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/components/ui/scrollFold.ts

@@ -10,7 +10,6 @@ import { CONTENT_ID } from "./landmarks";
 //    back (`room - saving > FOLD_AT`): a page barely taller than the window would otherwise fold, lose its
 //    scroll, land at the top, unfold, and so on, with every wheel tick;
 //  - unfold only at the very top, not at the fold point: between the two the header keeps whatever it was.
-/** @internal exported for scrollFold.test.ts */
 export const FOLD_AT = 32;
 
 interface ScrollPosition {
@@ -21,7 +20,6 @@ interface ScrollPosition {
 }
 
 // `saving` = the height the header gives back when it folds (measured while it is unfolded).
-/** @internal exported for scrollFold.test.ts */
 export function nextFolded(
   folded: boolean,
   { top, room }: ScrollPosition,