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/normalizationSnapshot.ts

@@ -0,0 +1,51 @@
+import { useAuthStore } from './authStore';
+import { resolveReplayGainDb } from '../utils/resolveReplayGainDb';
+import type { PlayerState, Track } from './playerStore';
+
+/**
+ * Compute the normalization fields that should land in the next state commit
+ * when the runtime switches tracks or rewrites the queue. Three branches:
+ *
+ *  - **loudness** — clear the visible dB until the engine pushes a real
+ *    `audio:normalization-state` event back; surface the user's target LUFS.
+ *  - **replaygain (enabled)** — derive the dB from track/album tags via
+ *    `resolveReplayGainDb`, add the pre-gain, fall back to the configured
+ *    fallback when no tag is available.
+ *  - **off** (or replaygain disabled) — everything null, engine `'off'`.
+ */
+export 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',
+  };
+}