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>

simulator/e2e/tests/audio-source.spec.ts

@@ -0,0 +1,73 @@
+import { test, expect, Page } from "@playwright/test";
+
+// The Audio-source feature (docs/superpowers/specs/2026-07-01-audio-music-option-design.md):
+// the old 0–10 "Audio" slider becomes "Volume", and a new Audio <select> chooses the
+// source — None / Soundtrack / Music. Music is ONE ethereal loop played the same at
+// every altitude (no per-altitude crossfade).
+
+async function enter(page: Page) {
+  await page.goto("/");
+  await page.waitForFunction(() => (window as any).__hefReady === true, null, { timeout: 45_000 });
+  await page.locator("#welcome-launch").click();
+  await page.waitForSelector("#welcome", { state: "detached", timeout: 10_000 });
+}
+
+async function selectSource(page: Page, v: string) {
+  await page.selectOption("#audio-source", v);
+}
+
+// The url the active <audio> element is loaded with (dataset.url is the raw path
+// app.js assigns; set regardless of whether play() is allowed to start).
+async function audUrls(page: Page): Promise<{ a: string; b: string }> {
+  return await page.evaluate(() => ({
+    a: (document.querySelector("#aud") as HTMLAudioElement).dataset.url || "",
+    b: (document.querySelector("#aud-b") as HTMLAudioElement).dataset.url || "",
+  }));
+}
+
+async function wheelOnStage(page: Page, deltaY: number) {
+  await page.locator("#stage").dispatchEvent("wheel", { deltaY });
+}
+
+test.describe("Audio source dropdown + Volume rename", () => {
+  test("the level slider is now labelled Volume and the Audio control is a 3-option dropdown", async ({ page }) => {
+    await enter(page);
+    await expect(page.locator('.panel label.audio-level span[data-i18n="output.volume"]')).toHaveText("Volume");
+    await expect(page.locator('.panel label.audio-source span[data-i18n="output.audio"]')).toHaveText("Audio");
+    const opts = page.locator("#audio-source option");
+    await expect(opts).toHaveCount(3);
+    await expect(opts).toHaveText(["None", "Soundtrack", "Music"]);
+  });
+
+  test("selecting None disables the Volume slider; a real source re-enables it", async ({ page }) => {
+    await enter(page);
+    await selectSource(page, "none");
+    await expect(page.locator("#audio")).toBeDisabled();
+    await selectSource(page, "soundtrack");
+    await expect(page.locator("#audio")).toBeEnabled();
+  });
+
+  test("Music plays one ethereal loop that persists across an altitude change", async ({ page }) => {
+    await enter(page);
+    await selectSource(page, "music");
+    let urls = await audUrls(page);
+    expect(urls.a).toContain("music/ethereal.loop.mp3");
+    // Move the dial: Music is altitude-independent, so #aud stays on the music loop
+    // (it does NOT switch to a per-altitude soundtrack).
+    await wheelOnStage(page, 120);
+    await wheelOnStage(page, 120);
+    await page.waitForTimeout(300);
+    urls = await audUrls(page);
+    expect(urls.a).toContain("music/ethereal.loop.mp3");
+  });
+
+  test("Soundtrack loads a per-altitude ambience, not the music loop", async ({ page }) => {
+    await enter(page);
+    await selectSource(page, "soundtrack");
+    await page.waitForTimeout(200);
+    const urls = await audUrls(page);
+    const loaded = urls.a + "|" + urls.b;
+    expect(loaded).toContain("/audio/");
+    expect(loaded).not.toContain("music/ethereal.loop.mp3");
+  });
+});