refactor(player): E.13 — extract loudness-backfill retry state (#576)

The two parallel maps that bound the per-track loudness backfill
retries (`analysisBackfillInFlightByTrackId`,
`analysisBackfillAttemptsByTrackId`) plus the `MAX_BACKFILL_ATTEMPTS_PER_TRACK`
constant and the `resetLoudnessBackfillStateForTrackId` reseed helper
move into `src/store/loudnessBackfillState.ts`. The new module exposes
a thin API:

  - `isBackfillInFlight` / `getBackfillAttempts` (reads)
  - `markBackfillInFlight(trackId, nextAttempt)` (atomic flag + counter)
  - `clearBackfillInFlight` (after promise settles)
  - `resetBackfillAttempts` (after refresh-hit)
  - `resetLoudnessBackfillStateForTrackId` (full reset across both id forms)

playerStore's five direct-access sites inside `refreshLoudnessForTrack`
become API calls; the mutables are no longer reachable from outside the
module. 13 focused tests pin atomicity, independence between tracks,
the partial-clear shapes (flag-only / counter-only), and the two-form
reseed expansion.

playerStore 3263 → 3261 LOC (small line delta because the inflight
flag + counter setup collapses to one call but the reset helper is no
longer inline).

src/store/loudnessBackfillState.test.ts

@@ -0,0 +1,91 @@
+/**
+ * Backfill state: two parallel maps that retry the per-track loudness
+ * analysis a bounded number of times. The interesting behaviours are the
+ * `markBackfillInFlight` atomicity (both flag + counter bump in one call)
+ * and the reseed reset that expands across the `stream:` / bare id forms
+ * via `loudnessCacheStateKeysForTrackId` (re-used from loudnessGainCache).
+ */
+import { afterEach, describe, expect, it } from 'vitest';
+import {
+  MAX_BACKFILL_ATTEMPTS_PER_TRACK,
+  _resetBackfillStateForTest,
+  clearBackfillInFlight,
+  getBackfillAttempts,
+  isBackfillInFlight,
+  markBackfillInFlight,
+  resetBackfillAttempts,
+  resetLoudnessBackfillStateForTrackId,
+} from './loudnessBackfillState';
+
+afterEach(() => {
+  _resetBackfillStateForTest();
+});
+
+describe('initial state', () => {
+  it('reports no inflight + 0 attempts for unknown tracks', () => {
+    expect(isBackfillInFlight('t1')).toBe(false);
+    expect(getBackfillAttempts('t1')).toBe(0);
+  });
+});
+
+describe('markBackfillInFlight', () => {
+  it('atomically sets inflight flag and counter', () => {
+    markBackfillInFlight('t1', 1);
+    expect(isBackfillInFlight('t1')).toBe(true);
+    expect(getBackfillAttempts('t1')).toBe(1);
+  });
+
+  it('keeps tracks independent', () => {
+    markBackfillInFlight('a', 1);
+    markBackfillInFlight('b', 2);
+    expect(getBackfillAttempts('a')).toBe(1);
+    expect(getBackfillAttempts('b')).toBe(2);
+    clearBackfillInFlight('a');
+    expect(isBackfillInFlight('a')).toBe(false);
+    expect(isBackfillInFlight('b')).toBe(true);
+  });
+});
+
+describe('clearBackfillInFlight', () => {
+  it('clears the flag without touching the counter', () => {
+    markBackfillInFlight('t1', 1);
+    clearBackfillInFlight('t1');
+    expect(isBackfillInFlight('t1')).toBe(false);
+    expect(getBackfillAttempts('t1')).toBe(1); // counter preserved
+  });
+});
+
+describe('resetBackfillAttempts', () => {
+  it('zeros the counter without touching the inflight flag', () => {
+    markBackfillInFlight('t1', 2);
+    resetBackfillAttempts('t1');
+    expect(getBackfillAttempts('t1')).toBe(0);
+    expect(isBackfillInFlight('t1')).toBe(true);
+  });
+});
+
+describe('MAX_BACKFILL_ATTEMPTS_PER_TRACK', () => {
+  it('is the hard-coded threshold the runtime uses', () => {
+    expect(MAX_BACKFILL_ATTEMPTS_PER_TRACK).toBe(2);
+  });
+});
+
+describe('resetLoudnessBackfillStateForTrackId', () => {
+  it('clears both maps for both id forms (bare + stream:)', () => {
+    markBackfillInFlight('t1', 1);
+    markBackfillInFlight('stream:t1', 2);
+    resetLoudnessBackfillStateForTrackId('t1');
+    expect(isBackfillInFlight('t1')).toBe(false);
+    expect(isBackfillInFlight('stream:t1')).toBe(false);
+    expect(getBackfillAttempts('t1')).toBe(0);
+    expect(getBackfillAttempts('stream:t1')).toBe(0);
+  });
+
+  it('also works when invoked with the stream-prefixed form', () => {
+    markBackfillInFlight('t1', 1);
+    markBackfillInFlight('stream:t1', 2);
+    resetLoudnessBackfillStateForTrackId('stream:t1');
+    expect(getBackfillAttempts('t1')).toBe(0);
+    expect(getBackfillAttempts('stream:t1')).toBe(0);
+  });
+});