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/readAhead.ts

@@ -8,16 +8,12 @@
 //   - ceiling MAX_LOOKAHEAD_ROWS: a tall window renders many rows, and an uncapped lookahead would
 //     eagerly prefetch several pages on every mount / filter change — wasted work on the app's
 //     heaviest query (the feed). The cap bounds that while still buffering a couple of screens ahead.
-/** @internal exported for readAhead.test.ts */
 export const PREFETCH_ROWS = 4;
-/** @internal exported for readAhead.test.ts */
 export const PREFETCH_VIEWPORTS = 2;
-/** @internal exported for readAhead.test.ts */
 export const MAX_LOOKAHEAD_ROWS = 40;
 
 /** How many rows ahead of the last rendered row to keep buffered, for a viewport rendering
  *  `renderedRows` rows. Clamped to [PREFETCH_ROWS, MAX_LOOKAHEAD_ROWS]. */
-/** @internal exported for readAhead.test.ts */
 export function readAheadLookahead(renderedRows: number): number {
   return Math.min(MAX_LOOKAHEAD_ROWS, Math.max(PREFETCH_ROWS, renderedRows * PREFETCH_VIEWPORTS));
 }