feat(audio): Audio source dropdown (None/Soundtrack/Music) + Volume rename

Split the single 0–10 'Audio' slider into two controls:
- Audio  — a dropdown selecting the source: None / Soundtrack / Music
- Volume — the renamed 0–10 slider (master gain for the active source)

Music is one ethereal, relaxing ambient loop played the same at every
altitude (no per-altitude crossfade), per the innerflo.me brief. Sourced
CC0: deadrobotmusic 'Ambient F Sharp Minor Ethereal Choir Pad' (freesound
#808032); added to build_audio_media.py, the candidate-pool doc, and the
public credits colophon. Media stays gitignored; run build_audio_media.py.

Routing lives in a pure, node-tested HEFScrub.audioPlan(source, level, frac);
app.js's restAudio/blendAudio consult it. None greys out Volume. The welcome
screen's mirror level slider is relabeled Volume too. i18n for all 4 langs.

Spec: docs/superpowers/specs/2026-07-01-audio-music-option-design.md
Tests: +5 node unit (audioPlan), +4 e2e (audio-source.spec.ts); 32 related
e2e green, 38 node unit green.

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

docs/audio-candidate-pool.md

@@ -9,8 +9,10 @@ model — random pick per landing, or pairable per-clip).
 The "audio + video" setting layers **two** kinds of audio per altitude:
 1. **Ambience / audio track** — the soundscape (waves, reef crackle, chorus, etc.).
    **This is what we're sourcing now.**
-2. **Music track** — a separate composed musical layer. **Deferred** until the
-   ambience tracks are settled across all altitudes.
+2. **Music track** — a separate composed musical layer. **Shipped** (2026-07-01)
+   as the Audio-dropdown **"Music"** option: unlike the earlier per-altitude idea,
+   it is now **one single ethereal loop played the same at every altitude** (see
+   § Music below).
 
 Pools below are the **ambience** layer unless marked `music`. **Distinction (operator):**
 the ambience/soundtrack layer is an *environmental, real-world soundscape* (waves, reef,
@@ -48,6 +50,22 @@ 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)
+
+**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
+stay as historical candidates). The operator's brief: *ethereal, relaxing — like
+innerflo.me.* CC0 (operator's sourcing choice), so attribution is courtesy, not required.
+
+| id | clip | source | dur | license |
+|---|---|---|---|---|
+| `music/ethereal` | **✓ KEEP** — "Ambient F Sharp Minor Ethereal Choir Pad 1": blurry, paulstretched layered choir — "peaceful, magical, soothing" | deadrobotmusic — https://freesound.org/people/deadrobotmusic/sounds/808032/ (hq preview: `cdn.freesound.org/previews/808/808032_*-hq.mp3`) | 3:38 | CC0 |
+
+Production: `simulator/build_audio_media.py` `SOURCES["music"]` → seamless loudness-normalized
+loop at `audio/music/ethereal.loop.mp3`, served `/media/audio/music/ethereal.loop.mp3`
+(client fallback: `app.js` `MUSIC_FALLBACK`). Download the sound page's WAV/MP3 to
+`simulator/sample_media/audio/music/ethereal.mp3`, then `python simulator/build_audio_media.py`.
+
 ## Sourcing rule (learned 2026-06-26)
 
 **Source only from a browsable page the operator can open and whose license is stated.**
@@ -67,7 +85,10 @@ hubs: NASA SVS / Chandra, NOAA PMEL/SanctSound (PD), BigSoundBank (CC0), Pixabay
 - **reef** — ✅ round-4 6-layer bake (`reef/soundscape`): NOAA PD biological layers + a CC0
   submerged underwater "you're under" bed (DCSFX freesound #366159, hq preview). License clean.
   SanctSound clips: `files/SanctSound_*.mp4` at `https://sanctsound.ioos.us/files/` (extract audio).
-- **music layer** — separate composed music track per altitude, deferred (cosmos slot reserved).
+- **music layer** — ✅ SHIPPED 2026-07-01 as the Audio-dropdown "Music" option, but as
+  ONE altitude-independent ethereal loop (`music/ethereal`, CC0), not the per-altitude
+  track once envisioned. See § Music above. (The `cosmos/music` / "Frozen Star" rows are
+  now historical per-altitude candidates, not planned work.)
 - **Production (after picks):** loop seams, length normalization, cross-altitude crossfade
   (mirror video); cosmos/abyss clips are short → loop.