feat(audio): rebuild playback + visualization on resident daemon and PCM cache

Two fragility points, rebuilt at the root:

Playback: one resident mpv daemon (--idle --keep-open) with a persistent
IPC connection and observe_property state instead of spawn-per-episode and
connect-per-poll. Play/pause/seek are sub-ms commands; time-pos pushes at
~20Hz; external pauses arrive as events. Boot session restore preloads the
episode paused (loadfile + paused time-pos seek, since mpv defers --start
stream work until playback) so first Play is a ~400ms unpause instead of a
cold 4.3s open+seek. Load ops are mutex-serialized so a raced preload
cannot clobber an in-flight play.

Data throttling: mpv demuxer cache capped (cache-secs=90, max-bytes=40MiB)
so a paused preload no longer races to its 150MiB default (measured
45.7MB/12s); decoder paced at 4x realtime instead of 84x so playback start
isn't starved by the visualizer ripping the whole episode.

Visualization: replaced the paced-ring reader (AudioStreamReader) with a
position-indexed PCM cache (audio-pcm-cache). ffmpeg fills a cache indexed
by absolute playback time; reads at the player position are always exact.
Pause freezes the render loop, resume re-arms it — no coverage guessing,
no clamped-buffer freeze (the pause->broken-waveform->freeze bug). Seeks
and speed changes need no pipeline restarts; uncovered reads return empty
and the last frame holds.

Cover art: persistent per-URL disk cache under XDG cache dir; play() no
longer awaits a curl subprocess (up to 8s). Cache hit = one stat; misses
apply late via mpv video-add.

Test suite: 161 pass. New tests pin the position-index contract (sample-
exact window reads, hold-on-uncovered, pause-keeps-cache, seek segments),
the daemon contract (play/pause/resume/seek/stop, preload fast path,
EOF->replay), and cover cache/single-flight/404.

tests/visualizer-store.test.ts

@@ -15,7 +15,7 @@
  *
  * Uses a self-generated local WAV (a frequency chirp, so different playback
  * positions produce measurably different bar output) and the real ffmpeg +
- * native cavacore pipeline, mirroring audio-stream-reader.test.ts.
+ * native cavacore pipeline, mirroring audio-pcm-cache.test.ts.
  *
  * Timing note: this is an integration test of the store's real timers — the
  * unload path is a genuine `setTimeout` in the store, and bun 1.3.8 ships no