refactor(player): E.18 — extract three transport-coordination modules (#581)

Cluster of three small thematically-related cuts in one PR (per the new
'cluster, don't single-shot' convention):

  - `src/store/seekTargetState.ts` — the seek-target guard (`seekTarget`,
    `seekTargetSetAt`, `SEEK_TARGET_GUARD_TIMEOUT_MS`, set/clear/get
    accessors) that suppresses stale Rust progress ticks until the
    engine catches up to the requested position
  - `src/store/togglePlayLock.ts` — the 300 ms double-click cooldown
    behind a `tryAcquireTogglePlayLock()` helper that auto-releases on
    a timer (collapses the three-line inline pattern in `togglePlay`)
  - `src/store/loudnessReseed.ts` — the full `reseedLoudnessForTrackId`
    pipeline (gen-bump → cache + backfill wipe → state reset → server
    row delete → forced seed enqueue), pulled out of playerStore as a
    single async helper

All three were file-private; no caller-side changes outside
playerStore's own imports. The progress handler's seek-guard branch is
now ~3 lines shorter and reads through accessors. `togglePlay` collapses
to one guard check.

24 tests across the three modules pin the API contracts.

playerStore 3189 → 3139 LOC.

src/store/loudnessReseed.ts

@@ -0,0 +1,66 @@
+import { invoke } from '@tauri-apps/api/core';
+import { buildStreamUrl } from '../api/subsonic';
+import { useAuthStore } from './authStore';
+import { usePlayerStore } from './playerStore';
+import { bumpWaveformRefreshGen } from './waveformRefreshGen';
+import { clearLoudnessCacheStateForTrackId } from './loudnessGainCache';
+import { resetLoudnessBackfillStateForTrackId } from './loudnessBackfillState';
+
+/**
+ * Tear down every cached piece of analysis for a track and re-enqueue a
+ * forced reseed. Used by the Settings „Re-analyse this track" action.
+ *
+ * Sequence:
+ *  1. Skip if loudness engine isn't active (re-analysis only makes sense
+ *     when normalization actually consumes the result).
+ *  2. Invalidate the waveform refresh generation so any in-flight read
+ *     for this id is discarded, and blank the seekbar bins immediately if
+ *     the track is current.
+ *  3. Wipe both loudness-cache maps (both forms of the id) + the backfill
+ *     retry state.
+ *  4. Reset the live normalization-state to placeholder values so the UI
+ *     doesn't show stale dB until the new analysis lands.
+ *  5. Delete the on-disk waveform + loudness rows via Rust.
+ *  6. Re-issue `updateReplayGainForCurrentTrack` so the engine drops
+ *     whatever gain it was holding for this track.
+ *  7. Enqueue a forced seed via `analysis_enqueue_seed_from_url`.
+ *
+ * Best-effort throughout — Rust errors are logged but never thrown.
+ */
+export async function reseedLoudnessForTrackId(trackId: string): Promise<void> {
+  if (!trackId) return;
+  const auth = useAuthStore.getState();
+  if (auth.normalizationEngine !== 'loudness') return;
+  bumpWaveformRefreshGen(trackId);
+  if (usePlayerStore.getState().currentTrack?.id === trackId) {
+    usePlayerStore.setState({ waveformBins: null });
+  }
+  clearLoudnessCacheStateForTrackId(trackId);
+  resetLoudnessBackfillStateForTrackId(trackId);
+  usePlayerStore.setState({
+    normalizationNowDb: null,
+    normalizationTargetLufs: auth.loudnessTargetLufs,
+    normalizationEngineLive: 'loudness',
+  });
+  try {
+    await invoke('analysis_delete_waveform_for_track', { trackId });
+  } catch (e) {
+    console.error('[psysonic] analysis_delete_waveform_for_track failed:', e);
+  }
+  try {
+    await invoke('analysis_delete_loudness_for_track', { trackId });
+  } catch (e) {
+    console.error('[psysonic] analysis_delete_loudness_for_track failed:', e);
+  }
+  usePlayerStore.getState().updateReplayGainForCurrentTrack();
+  const url = buildStreamUrl(trackId);
+  try {
+    await invoke('analysis_enqueue_seed_from_url', {
+      trackId,
+      url,
+      force: true,
+    });
+  } catch (e) {
+    console.error('[psysonic] analysis_enqueue_seed_from_url (reseed) failed:', e);
+  }
+}