refactor(player): E.6 — extract deriveNormalizationSnapshot into module (#569)

`deriveNormalizationSnapshot` — the loudness / replaygain / off branch
that the runtime uses to compute the normalization fields on every track
switch and queue rewrite — moves into src/store/normalizationSnapshot.ts.
File-private, four internal call sites updated to import from the new
module. Adds 8 focused tests pinning each branch (off, loudness with
target LUFS, replaygain enabled with tag + pre-gain, replaygain enabled
with fallback) plus neighbour-track context for album-mode resolution.

playerStore 3470 → 3434 LOC.

src/store/playerStore.ts

@@ -44,6 +44,7 @@ import {
   recordEnginePlayUrl,
   shouldRebindPlaybackToHotCache,
 } from './playbackUrlRouting';
+import { deriveNormalizationSnapshot } from './normalizationSnapshot';
 
 // Re-export the playback-progress public surface so existing call sites
 // (PlayerBar, FullscreenPlayer, WaveformSeek, LyricsPane, MobilePlayerView,
@@ -432,43 +433,6 @@ function emitNormalizationDebug(step: string, details?: Record<string, unknown>)
   }).catch(() => {});
 }
 
-function deriveNormalizationSnapshot(
-  track: Track,
-  queue: Track[],
-  queueIndex: number,
-): Pick<
-  PlayerState,
-  'normalizationNowDb' | 'normalizationTargetLufs' | 'normalizationEngineLive'
-> {
-  const auth = useAuthStore.getState();
-  const engine = auth.normalizationEngine;
-  if (engine === 'loudness') {
-    const target = auth.loudnessTargetLufs;
-    return {
-      // Clears stale UI until `audio:normalization-state` / refresh catches up.
-      normalizationNowDb: null,
-      normalizationTargetLufs: target,
-      normalizationEngineLive: 'loudness',
-    };
-  }
-  if (engine === 'replaygain' && auth.replayGainEnabled) {
-    const prev = queueIndex > 0 ? queue[queueIndex - 1] : null;
-    const next = queueIndex + 1 < queue.length ? queue[queueIndex + 1] : null;
-    const resolved = resolveReplayGainDb(track, prev, next, true, auth.replayGainMode);
-    const nowDb = resolved != null ? (resolved + auth.replayGainPreGainDb) : auth.replayGainFallbackDb;
-    return {
-      normalizationNowDb: nowDb,
-      normalizationTargetLufs: null,
-      normalizationEngineLive: 'replaygain',
-    };
-  }
-  return {
-    normalizationNowDb: null,
-    normalizationTargetLufs: null,
-    normalizationEngineLive: 'off',
-  };
-}
-
 // Debounce timer for seek slider drags.
 let seekDebounce: ReturnType<typeof setTimeout> | null = null;
 // Target time of the last seek — blocks stale Rust progress ticks until the