zen: calmer UB — live speed control (default 0.7×) + a touch bigger (scale 1.2)

Swim1_norm read a bit restless; add a tunable playback speed (default 0.7× for a calm
glide, ×0.7 again under reduced-motion) wired into the render tuner, and bump the default
scale to 1.2 for more presence. Both live-adjustable at /zen?debug=1.

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

frontend/src/routes/zen/+page.svelte

@@ -95,6 +95,8 @@
         <input type="range" min="-0.6" max="0.6" step="0.01" bind:value={dbg.pitch} oninput={apply} /></label>
       <label>scale <span>{dbg.scale.toFixed(2)}</span>
         <input type="range" min="0.3" max="2.5" step="0.05" bind:value={dbg.scale} oninput={apply} /></label>
+      <label>speed <span>{dbg.speed.toFixed(2)}</span>
+        <input type="range" min="0.2" max="1.5" step="0.05" bind:value={dbg.speed} oninput={apply} /></label>
 
       <hr />
       <div class="ph">Fins &amp; tail</div>