feat(static): "Run simulation" start button over the black stage

The experience boots silent and waits for a gesture (browser autoplay blocks
an un-gestured audio play()). Until now that gesture was an unlabelled Video
toggle — not obvious to a first-time viewer. Add a "Run simulation" button,
revealed over the black stage once media has preloaded; clicking it turns
Video on, snaps the audio dial to a gentle 2/10, dismisses the prompt, and
starts the soundtrack in that one gesture. Turning Video on directly still
begins the experience too (couples audio in, dismisses the button) via a
shared beginExperience(). Localised en/es/fr/ja.

E2E coverage updated (Playwright + pytest-playwright): button shown after
load, button starts video+audio+dismisses, and direct-Video-on path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

simulator/e2e/tests/altitude-lock.spec.ts

@@ -83,15 +83,27 @@ test("audio is a 0-10 level dial", async ({ page }) => {
   expect(ctl.max).toBe("10");
 });
 
-test("boots silent (video off, audio 0); turning video on couples audio to 3/10", async ({ page }) => {
+test("boots silent (video off, audio 0); turning video on couples audio to 2/10 and dismisses the Run prompt", async ({ page }) => {
   await boot(page);
   // Boots SILENT — no un-gestured auto-start, so the browser autoplay block never bites.
   expect(await page.evaluate(() => (document.querySelector("#visual") as HTMLInputElement).checked)).toBe(false);
   expect(await page.evaluate(() => (document.querySelector("#audio") as HTMLInputElement).value)).toBe("0");
-  // Turning Video on (a click gesture) lifts the audio dial to a gentle 3/10.
+  // "Run simulation" is revealed once preload finishes — the obvious starting point.
+  expect(await page.evaluate(() => document.querySelector("#run-sim")?.classList.contains("hidden"))).toBe(false);
+  // Turning Video on directly (a click gesture) lifts the audio dial to a gentle 2/10
+  // and dismisses the Run prompt.
   await enableVideo(page);
-  await expect.poll(() => page.evaluate(() => (document.querySelector("#audio") as HTMLInputElement).value)).toBe("3");
-  expect(await page.evaluate(() => document.querySelector("#audio-level-val")?.textContent)).toBe("3");
+  await expect.poll(() => page.evaluate(() => (document.querySelector("#audio") as HTMLInputElement).value)).toBe("2");
+  expect(await page.evaluate(() => document.querySelector("#audio-level-val")?.textContent)).toBe("2");
+  expect(await page.evaluate(() => document.querySelector("#run-sim")?.classList.contains("hidden"))).toBe(true);
+});
+
+test("the Run simulation button starts video, snaps audio to 2/10, and dismisses itself", async ({ page }) => {
+  await boot(page);
+  await page.click("#run-sim");
+  await expect.poll(() => page.evaluate(() => (document.querySelector("#visual") as HTMLInputElement).checked)).toBe(true);
+  await expect.poll(() => page.evaluate(() => (document.querySelector("#audio") as HTMLInputElement).value)).toBe("2");
+  expect(await page.evaluate(() => document.querySelector("#run-sim")?.classList.contains("hidden"))).toBe(true);
 });
 
 test("dragging the dial scrubs morph currentTime and audio gains", async ({ page }) => {

simulator/static/app.js

@@ -1239,6 +1239,23 @@ function hideLoading() {
 
 let videoEverOn = false;      // has Video ever been switched on this session?
 
+// The gentle audio level the experience starts at — coupled in on the first
+// Video-on (via the Run simulation button or the Video toggle directly).
+const START_AUDIO_LEVEL = "2";
+
+// Begin the experience: Video on, audio snapped to the gentle start level, and
+// the "Run simulation" prompt dismissed. Audio is set + played in THIS gesture so
+// it unlocks on Safari (which blocks play() outside a user gesture). One flip = the
+// full experience at a gentle level. Idempotent across re-toggles via videoEverOn.
+function beginExperience() {
+  videoEverOn = true;
+  const btn = $("run-sim");
+  if (btn) btn.classList.add("hidden");
+  $("audio").value = START_AUDIO_LEVEL;
+  updateAudioLevelLabel();
+  applyAudio();
+}
+
 async function main() {
   devLiveReload();
   try { initPaint(); } catch (e) { paintOK = false; paint.style.display = "none"; showError("WebGL init: " + e.message); }
@@ -1255,14 +1272,17 @@ async function main() {
   // play() outside a user gesture).
   $("audio").addEventListener("input", () => { updateAudioLevelLabel(); applyAudio(); debounced(); });
   updateAudioLevelLabel();
-  // Video toggle: the FIRST time video turns on, bring audio up to 3/10 with it (one
-  // flip = the full experience at a gentle level). If audio is already raised, leave it.
-  // Set + played in THIS gesture so it unlocks on Safari.
+  // "Run simulation" button: the obvious starting point. Turns Video on, snaps
+  // audio to the start level, and dismisses itself — all in this click gesture.
+  $("run-sim").addEventListener("click", () => {
+    $("visual").checked = true;   // setting .checked does NOT fire "change", so we drive the rest here
+    beginExperience();
+    debounced();
+  });
+  // Video toggle: the FIRST time video turns on (without the Run simulation button),
+  // begin the experience too — couple audio in and dismiss the prompt.
   $("visual").addEventListener("change", () => {
-    if ($("visual").checked && !videoEverOn) {
-      videoEverOn = true;
-      if (audioLevel() === 0) { $("audio").value = "3"; updateAudioLevelLabel(); applyAudio(); }
-    }
+    if ($("visual").checked && !videoEverOn) beginExperience();
     debounced();
   });
   // Altitude knob: drag to turn (commit detents on release), scroll to step, tap a label to jump.
@@ -1274,11 +1294,13 @@ async function main() {
   update();                   // render the initial state (both toggles off → black, silent)
   await preloadAllMedia();    // download all media, updating the loading bar
   hideLoading();              // experience is ready — boots SILENT (video off, audio 0)
+  $("run-sim").classList.remove("hidden");   // reveal the obvious starting point over the black stage
   // No un-gestured auto-start: a browser autoplay policy blocks an audio play() made
   // outside a user gesture (muted video survives, sound does not), so an auto-start
   // would show video but stay silent. Instead the experience waits for the operator to
-  // turn Video on — that click IS the gesture, and its handler (above) lifts audio to
-  // 3/10 and starts the soundtrack in-gesture, so sound reliably comes up with video.
+  // press "Run simulation" (or turn Video on directly) — that click IS the gesture, and
+  // beginExperience() lifts audio to the start level and plays the soundtrack in-gesture,
+  // so sound reliably comes up with video.
 }
 
 // Populate the language dropdown from the registry and wire live switching.

simulator/static/i18n.js

@@ -19,6 +19,7 @@
   const UI_STRINGS = {
     "app.title":      { en: "Human Experience Filter — Alteration Preview", es: "Filtro de Experiencia Humana — Vista previa de alteración", fr: "Filtre d’Expérience Humaine — Aperçu d’altération", ja: "ヒューマン・エクスペリエンス・フィルター — 変容プレビュー" },
     "loading.title":  { en: "Loading Universe", es: "Cargando el universo", fr: "Chargement de l’univers", ja: "宇宙を読み込み中" },
+    "run.button":     { en: "Run simulation", es: "Iniciar simulación", fr: "Lancer la simulation", ja: "シミュレーションを開始" },
     "output.legend":  { en: "Output", es: "Salida", fr: "Sortie", ja: "出力" },
     "output.video":   { en: "Video", es: "Vídeo", fr: "Vidéo", ja: "映像" },
     "output.audio":   { en: "Audio", es: "Audio", fr: "Audio", ja: "音声" },

simulator/static/index.html

@@ -26,6 +26,7 @@
         <svg id="overlay" viewBox="0 0 100 100" preserveAspectRatio="none"></svg>
         <svg id="affect" viewBox="0 0 100 100" preserveAspectRatio="none"></svg>
         <div id="black" class="black hidden"></div>
+        <button type="button" id="run-sim" class="run-sim hidden" data-i18n="run.button">Run simulation</button>
       </div>
     </section>
 

simulator/static/style.css

@@ -51,6 +51,20 @@ main { display: flex; gap: 1rem; padding: 1rem; flex-wrap: wrap; align-items: fl
 .black { position: absolute; inset: 0; background: #000; opacity: 1;
          z-index: 50; transform: translateZ(0); transition: opacity 200ms ease; }
 .hidden { display: none; }
+/* "Run simulation" — the obvious starting point, shown over the (black) stage once
+   media is preloaded. Sits above the black cover (z-index 50). Dismissed when the
+   experience begins (the button, or turning Video on directly). */
+.run-sim {
+  position: absolute; top: 50%; left: 50%; transform: translate(-50%, -50%);
+  z-index: 60; cursor: pointer; user-select: none;
+  padding: 0.85rem 2rem; border: 0; border-radius: 999px;
+  font: 600 18px/1 system-ui, sans-serif; letter-spacing: 0.03em;
+  color: #04101f; background: linear-gradient(90deg, #4e9cff, #9af);
+  box-shadow: 0 4px 24px rgba(78, 156, 255, 0.45);
+  transition: transform 0.12s ease, box-shadow 0.12s ease;
+}
+.run-sim:hover { transform: translate(-50%, -50%) scale(1.04); box-shadow: 0 6px 30px rgba(78, 156, 255, 0.6); }
+.run-sim:active { transform: translate(-50%, -50%) scale(0.98); }
 /* Stack the two Output toggles (Video / Audio) with a little breathing room. */
 .dev-switch + .dev-switch { margin-top: 0.45rem; }
 /* Live audio status readout (diagnostic) — turns green when actually playing. */