feat(player): waveform pipeline survives tab switches — 30s unload grace + braille spinner loading

The waveform's ffmpeg decode + cavacore FFT pipeline lived inside
RealtimeWaveform, so switching away from the Player tab unmounted it and
killed the pipeline instantly — respawning ffmpeg on every return.

Move the pipeline into a module-level store (stores/visualizer.ts) that
outlives the page:
- losing Player-tab focus keeps the pipeline warm for
  VISUALIZER_UNLOAD_DELAY_MS (30s), then tears it down (kills ffmpeg,
  destroys the cava plan); regaining focus within the delay resumes the
  warm pipeline with no restart churn; after an unload it restarts from
  the current playback position.
- playback signals move to utils/audio-signals.ts (module-level, no
  useAudio() owner needed) so the store reacts to play/pause/seek/speed
  while no Player page is mounted; useAudio re-imports them.
- render a braille spinner as the loading state for the visualizer
  (first play / after unload); stale bars stay on screen during warmup
  restarts so the waveform never blanks out for the network-bound cold
  start.
- seed the smooth position clock at pipeline start: with the position
  still frozen at 0 while mpv opens the stream, the reader sampled a
  1-sample window that could never fill, starving the bars until mpv's
  first poll.

Pins the store contract in tests/visualizer-store.test.ts: loading→bars,
warm resume without restart, 30s unload, and bars while position is
frozen at 0.

src/pages/Player/PlayerPage.tsx

@@ -10,12 +10,12 @@
  * tab root.
  */
 
-import { Show } from "solid-js";
+import { Show, onMount, onCleanup } from "solid-js";
 import { PlaybackControls } from "./PlaybackControls";
 import { ProgressBar } from "./ProgressBar";
 import { RealtimeWaveform } from "./RealtimeWaveform";
 import { useAudio } from "@/hooks/useAudio";
-import { useAppStore } from "@/stores/app";
+import { useVisualizer } from "@/stores/visualizer";
 import { useTheme } from "@/context/ThemeContext";
 import { useNavigation, DEPTH_CENTER_PANE } from "@/context/NavigationContext";
 import { PaneRow } from "@/components/PaneRow";
@@ -27,8 +27,16 @@ export function PlayerPage() {
 	const audio = useAudio();
 	const { theme } = useTheme();
 	const nav = useNavigation();
+	const viz = useVisualizer();
 	const muted = () => theme.muted || theme.text;
 
+	// The page is mounted exactly while the Player tab is in focus (Shell
+	// renders only the active tab), so mount ⇔ focused. Report it to the
+	// visualizer store: losing focus starts the unload grace timer instead
+	// of killing the pipeline with the page; regaining focus restarts it.
+	onMount(() => viz.setFocused(true));
+	onCleanup(() => viz.setFocused(false));
+
 	const isActive = () => nav.activePane() === DEPTH_CENTER_PANE;
 
 	const progressPercent = () => {
@@ -82,18 +90,7 @@ export function PlayerPage() {
 
 						<ProgressBar />
 
-						<RealtimeWaveform
-							visualizerConfig={(() => {
-								const viz = useAppStore().state().settings.visualizer;
-								// bars is width-derived in RealtimeWaveform; pass only the
-								// audio-processing params here.
-								return {
-									noiseReduction: viz.noiseReduction,
-									lowCutOff: viz.lowCutOff,
-									highCutOff: viz.highCutOff,
-								};
-							})()}
-						/>
+						<RealtimeWaveform />
 					</box>
 				)}
 			</Show>