feat(audio): shareable ?audio= deep-link presets the sound mode

?audio=none|soundtrack|music (and ?vol=0..10) presets the Audio source/volume on
load, so a link opens straight into a chosen sound mode — e.g. share …/?audio=music
with someone who'd prefer music, …/?audio=soundtrack for the ambience. Invalid
values ignored (default Soundtrack stands). +5 e2e (url-params.spec.ts).

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

simulator/e2e/tests/url-params.spec.ts

@@ -0,0 +1,47 @@
+import { test, expect, Page } from "@playwright/test";
+
+// Shareable deep-link: ?audio=none|soundtrack|music presets the Audio source on load
+// (and ?vol=0..10 the master volume), so a link opens straight into a chosen sound mode.
+
+async function ready(page: Page) {
+  await page.waitForFunction(() => (window as any).__hefReady === true, null, { timeout: 45_000 });
+}
+async function launch(page: Page) {
+  await page.locator("#welcome-launch").click();
+  await page.waitForSelector("#welcome", { state: "detached", timeout: 10_000 });
+}
+const srcVal = (page: Page) => page.locator("#audio-source").inputValue();
+
+test("?audio=music preselects the Music source and survives launch", async ({ page }) => {
+  await page.goto("/?audio=music");
+  await ready(page);
+  expect(await srcVal(page)).toBe("music");   // set at init, before launch
+  await launch(page);
+  expect(await srcVal(page)).toBe("music");
+});
+
+test("?audio=soundtrack preselects Soundtrack", async ({ page }) => {
+  await page.goto("/?audio=soundtrack");
+  await ready(page);
+  expect(await srcVal(page)).toBe("soundtrack");
+});
+
+test("?audio=none preselects None and disables Volume", async ({ page }) => {
+  await page.goto("/?audio=none");
+  await ready(page);
+  expect(await srcVal(page)).toBe("none");
+  await expect(page.locator("#audio")).toBeDisabled();
+});
+
+test("an invalid ?audio value is ignored (stays default Soundtrack)", async ({ page }) => {
+  await page.goto("/?audio=banana");
+  await ready(page);
+  expect(await srcVal(page)).toBe("soundtrack");
+});
+
+test("?vol sets the master volume for the shared link", async ({ page }) => {
+  await page.goto("/?audio=music&vol=7");
+  await ready(page);
+  await launch(page);
+  expect(await page.locator("#audio").inputValue()).toBe("7");
+});

simulator/static/app.js

@@ -1410,6 +1410,27 @@ function syncVolumeDisabled() {
   if (slider) slider.disabled = audioSource() === "none";
 }
 
+// Shareable deep-link: ?audio=none|soundtrack|music presets the Audio source on load
+// (and ?vol=0..10 the master volume), so a link opens straight into a chosen sound mode
+// — e.g. share …/?audio=music with someone who'd prefer music over the soundtrack. The
+// source is set on the (welcome-hidden) panel dropdown; the launch gesture then applies
+// it. Invalid values are ignored (the default Soundtrack stands).
+function applyUrlAudioParams() {
+  const p = new URLSearchParams(location.search);
+  const src = (p.get("audio") || "").toLowerCase();
+  if (src === "none" || src === "soundtrack" || src === "music") {
+    const el = $("audio-source"); if (el) el.value = src;
+  }
+  const vol = p.get("vol");
+  if (vol !== null && vol !== "" && Number.isFinite(+vol)) {
+    const v = String(Math.max(0, Math.min(10, Math.round(+vol))));
+    // Seed BOTH the welcome mirror (carried into the panel on launch) and the panel
+    // slider, so the shared level survives entry.
+    const w = $("welcome-audio"); if (w) w.value = v;
+    const a = $("audio"); if (a) a.value = v;
+  }
+}
+
 // The element currently carrying the most signal — for the diagnostic readout,
 // which used to assume a single `aud`. With two crossfading elements either may be
 // the audible one, so report on whichever is louder (ties → aud).
@@ -1682,7 +1703,9 @@ async function main() {
   // Audio source dropdown: reconcile SYNCHRONOUSLY in this change gesture so
   // switching to Soundtrack/Music unlocks + plays on Safari; None greys Volume.
   $("audio-source").addEventListener("change", () => { syncVolumeDisabled(); applyAudio(); debounced(); });
+  applyUrlAudioParams();        // shareable ?audio=/?vol= deep-link presets
   syncVolumeDisabled();
+  updateWelcomeAudioLabel();
   // Video toggle (panel): the FIRST time video turns on without having entered via
   // Launch, begin the experience too — couple audio in. (Normally Launch entered
   // first, so videoEverOn is already true and this is a no-op beyond re-render.)