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/layerStack.ts

@@ -48,7 +48,6 @@ export function anyBlockingLayer(): boolean {
   return stack.some((id) => !passive.has(id));
 }
 
-/** @internal exported for layerStack.dom.test.ts */
 export function isTopmost(id: number): boolean {
   return stack.length > 0 && stack[stack.length - 1] === id;
 }
@@ -79,7 +78,6 @@ export function anyBlockingLayerAbove(id: number): boolean {
 // mutation; `anyLayerAbove` stays as the fallback for a layer above with no Escape handler of its own.
 // WeakSet so a spent event is GC'd without a manual sweep.
 const handledEscapes = new WeakSet<KeyboardEvent>();
-/** @internal exported for keymap.test.ts, layerStack.dom.test.ts */
 export function markEscapeHandled(e: KeyboardEvent): void {
   handledEscapes.add(e);
 }