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>

tests/test_e2e_audio.py

@@ -7,10 +7,10 @@ blanking) — real Safari/iOS autoplay and real-GPU compositing still need a dev
 by-ear/eye check.
 
 The experience BOOTS SILENT — video off, audio dial at 0 — so nothing plays until a
-real user gesture. Turning Video ON (a click) couples the audio dial up to 3/10 and
-starts the soundtrack IN that gesture, sidestepping the browser autoplay block that
-swallowed an un-gestured auto-start. Audio is a 0–10 range dial (not a checkbox);
-set it by writing `value` + firing an `input` event.
+real user gesture. The "Run simulation" button (or turning Video ON directly) couples
+the audio dial up to 2/10 and starts the soundtrack IN that gesture, sidestepping the
+browser autoplay block that swallowed an un-gestured auto-start. Audio is a 0–10 range
+dial (not a checkbox); set it by writing `value` + firing an `input` event.
 
 Install:  pip install -e '.[e2e]' && python -m playwright install chromium
 """
@@ -100,12 +100,36 @@ def test_app_boots_silent_video_off(page):
     ) is True
 
 
-def test_video_on_couples_audio_to_three_and_plays(page):
-    # Turning Video on (a click gesture) lifts the audio dial to 3/10 and starts the
-    # soundtrack IN that gesture: video shows (black hidden), a scale soundtrack plays.
+def test_run_simulation_button_shows_after_load(page):
+    # Once media is preloaded the "Run simulation" prompt is revealed over the (still
+    # black) stage — the obvious starting point — while the experience stays silent.
+    page.wait_for_selector("#run-sim:not(.hidden)")
+    assert page.is_checked("#visual") is False
+    assert page.evaluate("document.getElementById('audio').value") == "0"
+
+
+def test_run_simulation_button_starts_video_and_audio(page):
+    # Pressing "Run simulation" (a click gesture) turns Video on, snaps the audio dial
+    # to 2/10, dismisses itself, and starts the soundtrack IN that gesture.
+    page.click("#run-sim")
+    page.wait_for_selector("#run-sim.hidden", state="attached")        # prompt dismissed
+    assert page.is_checked("#visual") is True
+    page.wait_for_function("document.getElementById('audio').value === '2'")
+    page.wait_for_function("document.getElementById('audio-level-val').textContent === '2'")
+    page.wait_for_selector("#black.hidden", state="attached")          # video showing
+    page.wait_for_function(
+        "(() => { const a = document.getElementById('aud');"
+        " return /\\/media\\/audio\\/.+\\.mp3/.test(a.src) && !a.paused; })()"
+    )
+
+
+def test_video_on_directly_couples_audio_to_two_and_dismisses_button(page):
+    # Turning Video on directly (skipping the Run simulation button) ALSO begins the
+    # experience: audio snaps to 2/10, the soundtrack plays, and the button goes away.
     _toggle_visual(page)                                                # off -> on
-    page.wait_for_function("document.getElementById('audio').value === '3'")
-    page.wait_for_function("document.getElementById('audio-level-val').textContent === '3'")
+    page.wait_for_function("document.getElementById('audio').value === '2'")
+    page.wait_for_function("document.getElementById('audio-level-val').textContent === '2'")
+    page.wait_for_selector("#run-sim.hidden", state="attached")        # prompt dismissed
     page.wait_for_selector("#black.hidden", state="attached")          # video showing
     page.wait_for_function(
         "(() => { const a = document.getElementById('aud');"