refactor(player): E.19 — extract analysis-refresh helpers cluster (#583)

Two thematic cuts in one PR:

  - `src/store/waveformRefresh.ts` — `refreshWaveformForTrack` plus its
    `WaveformCachePayload` type. Fetches the cached waveform row and
    applies bins to the player store, guarded by both the refresh
    generation snapshot and the current-track check so a stale read
    can't overwrite fresh data.
  - `src/store/loudnessRefresh.ts` — `refreshLoudnessForTrack` plus the
    `loudnessRefreshInflight` coalescing map and `LoudnessCachePayload`
    type. Orchestrates the loudness fetch: dedupe concurrent calls by
    (trackId, syncEngine, target), distinguish hit vs miss, enqueue
    bounded backfill, suppress stale-target results by recursive retry.

Both file-private; no caller-side changes outside playerStore's own
imports. Imports of helpers that were only used by these two functions
get cleaned up out of playerStore (coerceWaveformBins, getBackfillAttempts,
forgetLoudnessGain, redactSubsonicUrlForLog, LOUDNESS_BACKFILL_WINDOW_AHEAD,
isTrackInsideLoudnessBackfillWindow, etc.).

20 tests across the two modules pin the orchestration: gen + current-track
guards on waveform; coalesce + hit/miss/backfill/stale-target/sync-flag
branches on loudness.

playerStore 3139 → 2983 LOC — first sub-3000 milestone for Phase E.

src/store/waveformRefresh.ts

@@ -0,0 +1,45 @@
+import { invoke } from '@tauri-apps/api/core';
+import { coerceWaveformBins } from '../utils/waveformParse';
+import { usePlayerStore } from './playerStore';
+import { getWaveformRefreshGen } from './waveformRefreshGen';
+
+/** Subsonic-server waveform-cache row as Rust hands it back. */
+export type WaveformCachePayload = {
+  /** May be `number[]` or `Uint8Array` depending on Tauri IPC / serde path. */
+  bins: number[] | Uint8Array;
+  binCount: number;
+  isPartial: boolean;
+  knownUntilSec: number;
+  durationSec: number;
+  updatedAt: number;
+};
+
+/**
+ * Fetch the cached waveform row for `trackId` from Rust and apply its bins
+ * to the player store — but only if (a) the refresh generation snapshot
+ * still matches (no newer invalidation has fired meanwhile) and (b) the
+ * track is still the current one. Best-effort: any failure leaves the
+ * seekbar with the placeholder waveform.
+ */
+export async function refreshWaveformForTrack(trackId: string): Promise<void> {
+  if (!trackId) return;
+  const gen = getWaveformRefreshGen(trackId);
+  try {
+    const row = await invoke<WaveformCachePayload | null>('analysis_get_waveform_for_track', { trackId });
+    if (getWaveformRefreshGen(trackId) !== gen) return;
+    // Never apply bins for a non-current track (e.g. gapless byte-preload fetches the neighbour).
+    if (usePlayerStore.getState().currentTrack?.id !== trackId) return;
+    const bins = row ? coerceWaveformBins(row.bins) : null;
+    if (!bins || bins.length === 0) {
+      usePlayerStore.setState({
+        waveformBins: null,
+      });
+      return;
+    }
+    usePlayerStore.setState({
+      waveformBins: bins,
+    });
+  } catch {
+    // best-effort; seekbar falls back to placeholder waveform
+  }
+}