refactor(player): E.3 — extract waveform/normalization/seek pure helpers (#566)

Three small tranches out of playerStore.ts:
- src/utils/waveformParse.ts — waveformBlobLenOk + coerceWaveformBins (the
  parser that handles number[] / Uint8Array / ArrayLike payloads Rust
  serializes as).
- src/utils/normalizationCompare.ts — normalizationAlmostEqual (tolerant
  null-aware dB comparison).
- src/utils/seekErrors.ts — isRecoverableSeekError (retry classifier for
  the Rust seek pipeline).

Each gets focused unit tests. All four were file-private, no external
callers — pure code move. playerStore 3556 → 3516 LOC.

src/utils/normalizationCompare.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, it } from 'vitest';
+import { normalizationAlmostEqual } from './normalizationCompare';
+
+describe('normalizationAlmostEqual', () => {
+  it('returns true for two nulls', () => {
+    expect(normalizationAlmostEqual(null, null)).toBe(true);
+  });
+
+  it('returns false for mixed null / number', () => {
+    expect(normalizationAlmostEqual(null, 0)).toBe(false);
+    expect(normalizationAlmostEqual(0, null)).toBe(false);
+  });
+
+  it('returns true for values within the default epsilon (0.12)', () => {
+    expect(normalizationAlmostEqual(-14.0, -14.05)).toBe(true);
+    expect(normalizationAlmostEqual(-14.0, -14.12)).toBe(true);
+  });
+
+  it('returns false just past the default epsilon', () => {
+    expect(normalizationAlmostEqual(-14.0, -14.13)).toBe(false);
+  });
+
+  it('honours a custom epsilon', () => {
+    expect(normalizationAlmostEqual(-14.0, -14.5, 1.0)).toBe(true);
+    expect(normalizationAlmostEqual(-14.0, -14.5, 0.4)).toBe(false);
+  });
+
+  it('treats exact equality as equal', () => {
+    expect(normalizationAlmostEqual(-10, -10)).toBe(true);
+    expect(normalizationAlmostEqual(0, 0)).toBe(true);
+  });
+});