refactor(player): E.24 — extract radio session + infinite-queue guards (#588)

Cluster of four small mutables in two modules:

  - `src/store/radioSessionState.ts` — `radioFetching` (concurrent
    fetch guard) + `currentRadioArtistId` (seed artist that survives
    track advances) + `radioSessionSeenIds` (dedupe set including
    HISTORY_KEEP-evicted entries, fixes issue #500).
  - `src/store/infiniteQueueState.ts` — `infiniteQueueFetching`
    concurrent fetch guard.

Sed-driven bulk rewrite for the >35 direct-access sites. The four
`= new Set()` resets become `clearRadioSessionSeenIds()` calls and
the `setRadioArtistId` / `enqueueRadio` actions read through
`getCurrentRadioArtistId()` instead of touching the mutable directly.

15 focused tests across the two modules.

playerStore 2867 → 2865 LOC.

src/store/radioSessionState.test.ts

@@ -0,0 +1,76 @@
+import { afterEach, describe, expect, it } from 'vitest';
+import {
+  _resetRadioSessionStateForTest,
+  addRadioSessionSeen,
+  clearRadioSessionSeenIds,
+  deleteRadioSessionSeen,
+  getCurrentRadioArtistId,
+  hasRadioSessionSeen,
+  isRadioFetching,
+  setCurrentRadioArtistId,
+  setRadioFetching,
+} from './radioSessionState';
+
+afterEach(() => {
+  _resetRadioSessionStateForTest();
+});
+
+describe('radioFetching', () => {
+  it('starts false + round-trips through set/get', () => {
+    expect(isRadioFetching()).toBe(false);
+    setRadioFetching(true);
+    expect(isRadioFetching()).toBe(true);
+    setRadioFetching(false);
+    expect(isRadioFetching()).toBe(false);
+  });
+});
+
+describe('currentRadioArtistId', () => {
+  it('starts null + round-trips', () => {
+    expect(getCurrentRadioArtistId()).toBeNull();
+    setCurrentRadioArtistId('artist-1');
+    expect(getCurrentRadioArtistId()).toBe('artist-1');
+    setCurrentRadioArtistId(null);
+    expect(getCurrentRadioArtistId()).toBeNull();
+  });
+});
+
+describe('radioSessionSeenIds', () => {
+  it('starts empty', () => {
+    expect(hasRadioSessionSeen('any')).toBe(false);
+  });
+
+  it('add + has round-trip', () => {
+    addRadioSessionSeen('t1');
+    expect(hasRadioSessionSeen('t1')).toBe(true);
+    expect(hasRadioSessionSeen('t2')).toBe(false);
+  });
+
+  it('delete removes individual ids without affecting others', () => {
+    addRadioSessionSeen('t1');
+    addRadioSessionSeen('t2');
+    deleteRadioSessionSeen('t1');
+    expect(hasRadioSessionSeen('t1')).toBe(false);
+    expect(hasRadioSessionSeen('t2')).toBe(true);
+  });
+
+  it('clearRadioSessionSeenIds wipes the set', () => {
+    addRadioSessionSeen('t1');
+    addRadioSessionSeen('t2');
+    clearRadioSessionSeenIds();
+    expect(hasRadioSessionSeen('t1')).toBe(false);
+    expect(hasRadioSessionSeen('t2')).toBe(false);
+  });
+});
+
+describe('_resetRadioSessionStateForTest', () => {
+  it('resets all three pieces of state', () => {
+    setRadioFetching(true);
+    setCurrentRadioArtistId('artist-1');
+    addRadioSessionSeen('t1');
+    _resetRadioSessionStateForTest();
+    expect(isRadioFetching()).toBe(false);
+    expect(getCurrentRadioArtistId()).toBeNull();
+    expect(hasRadioSessionSeen('t1')).toBe(false);
+  });
+});