feat(audio): Music becomes a crossfading playlist of 5 mellow synth tracks

Replace the single ethereal loop with the operator's chosen set — Music for
Manatees, Dewdrop Fantasy, Eastminster, Overheat, Concentration (Kevin MacLeod,
CC BY 4.0, drumless). The Music source now plays one track and every 5 min
crossfades to a random OTHER track (pure HEFScrub.pickNextIndex, never an
immediate repeat), altitude-independent, reusing the two audio elements; both
primed in-gesture for Safari. Intervals injectable via window.__hefMusicRotateMs
/ __hefMusicXfadeMs.

Media loudness-normalized + transcoded under the 25MB LFS limit (build_audio_media
MUSIC), committed via LFS; ethereal loop retired. CC-BY attribution added to
credits.html. Docs updated (music-candidate-pool = final set; audio-candidate-pool
ethereal section superseded).

+2 node unit (pickNextIndex), +2 e2e (music-playlist), audio-source spec updated.
40 unit + 49 e2e green; live probe confirms playback + rotation.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/audio-candidate-pool.md

@@ -50,7 +50,13 @@ recordings (NASA/NOAA) keep the "machines reshaping perception" thesis literal.
 | 🏖 coast | `coast/waves` | Sea waves + tern calls | BigSoundBank #0267 — https://bigsoundbank.com/sea-waves-and-seagulls-s0267.html (`/UPLOAD/mp3/0267.mp3`) | 57s | CC0 |
 | 🕳 abyss | `abyss/whale` | NE Pacific blue-whale "AB call" | NOAA PMEL — https://www.pmel.noaa.gov/acoustics/multimedia/NETS_bluWhale.wav | 13s | PD (NOAA) |
 
-## Music (the Audio-dropdown "Music" option, shipped 2026-07-01)
+## Music (the Audio-dropdown "Music" option)
+
+> **SUPERSEDED 2026-07-02:** the single ethereal loop below was replaced by a
+> **rotating crossfading playlist of mellow drumless synth** (Kevin MacLeod, CC BY) —
+> see `docs/music-candidate-pool.md`. The ethereal-loop record is kept for history.
+
+### (history) ethereal loop — shipped 2026-07-01, then replaced
 
 **One** ethereal ambient loop, played the same at every altitude (altitude-independent —
 NOT the per-altitude idea the "cosmos/music" / "Frozen Star" rows above reserved; those

docs/music-candidate-pool.md

@@ -12,13 +12,22 @@ URL). Local review gallery (gitignored, like the other review pages):
 **Honesty flag:** licenses + URLs are verified; the "harmony" notes are inferred from
 genre/reputation, not a by-ear check — audition before final selection.
 
-## Operator picks & refined brief (2026-07-02)
+## FINAL selection — WIRED 2026-07-02
 
-Operator auditioned and **kept `dreamcult` (Dream Culture) + `silverblue` (Silver Blue Light)**
-— both Kevin MacLeod, CC BY, drumless cinematic-calm synth. **Rejected everything else** as
-"boring OR too much snare drum." So the refined target is **drumless, evolving-chord ambient
-synth** in that exact lane — the HoliznaCC0 lo-fi (beat-driven) is OUT. A Round-4 search is
-sourcing ~8–12 more like the two keepers (Kevin MacLeod's Calming/Ambient catalog first).
+After a Round-4 drumless pass, the operator's **final Music set = 5 Kevin MacLeod tracks**
+(CC BY 4.0), now **shipped as a rotating crossfading playlist**:
+
+**Music for Manatees · Dewdrop Fantasy · Eastminster · Overheat · Concentration**
+
+Wiring: `app.js` `MUSIC_PLAYLIST` — the Music source plays one track and every
+`MUSIC_ROTATE_MS` (5 min) crossfades to a random OTHER track (`pickNextIndex`, never an
+immediate repeat), altitude-independent, on the two `#aud`/`#aud-b` elements. Files
+loudness-normalized + transcoded under the 25MB LFS limit by `build_audio_media.py`
+(`MUSIC`), committed via LFS; the single ethereal loop was **retired**. Credited on
+`credits.html` (CC BY attribution). Earlier candidates below kept for record.
+
+*(History: the operator first kept Dream Culture + Silver Blue Light, then — after the
+drumless Round-4 pass — chose the 5 long-form/floaty tracks above instead.)*
 
 ## Two license families