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/playerVolume.ts

@@ -19,7 +19,6 @@ export interface VolumeScale {
 
 /**
  * YouTube iframe API: 0–100 integers, no mute flag (level 0 IS muted).
- * @internal exported for playerVolume.test.ts
  */
 export const YT_SCALE: VolumeScale = { min: 0, max: 100, default: 100, round: true };
 /** HTML5 <video>: a 0..1 float (mute is a separate flag the caller carries). */