zen: fluid rework — slow banked U-turns synced to the clip + calmer cruise + clip preview

Phase-B first pass was stiff/awkward — the turn whipped ~180° in ~0.7s while the turn clip
ran 2.5s (fighting), and a flat fish spun that fast went edge-on. Rework:
- U-turn now runs over the clip's duration (~2.4s), smootherstep ease-in-out, banks in/out
  (roll), glides the arc instead of stopping — heading + animation move together.
- Calmer: slower cruise (0.26), fewer modes (cruise/rest, dropped jerky darts), longer
  timers, gentle continuous roll/pitch body sway so cruising isn't rigid.
- New "preview clip" tuner control: freeze locomotion + loop any raw clip broadside — proves
  the animation itself is fluid (isolates engine vs asset).

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

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

@@ -89,6 +89,12 @@
     <div class="panel">
       <div class="prow"><strong>UB render tuner</strong><button class="copy" onclick={copyValues}>{copied ? 'copied ✓' : 'copy values'}</button></div>
 
+      <label>preview clip
+        <select bind:value={dbg.preview} onchange={apply}>
+          <option value="">— live behavior —</option>
+          <option>idle</option><option>cruise</option><option>burst</option><option>turnL</option><option>turnR</option>
+        </select></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>