zen: UB base loop → Swim1_norm (gentle in-place swim, not the static Idle)

Idle is a resting pose (only eye/mouth/fin micro-motion). Swapped the base loop to
Swim1_norm — a ~2.5s swim cycle with ZERO root drift (verified via trim-clip.mjs), so
UB undulates continuously without traveling off-screen. Generalized the trimmer
(tools/glb-split/trim-clip.mjs: extract any [start,end] range, rebased to 0, + drift report).

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

frontend/src/lib/zen/aquarium.js

@@ -19,7 +19,8 @@ import { GLTFLoader } from 'three/addons/loaders/GLTFLoader.js';
 const MODEL_URL = '/models/ub-angelfish.glb';
 
 // UB is now a marine angelfish (Queen): ONE mesh, TWO materials (…_body, …_fins — fins
-// covers the caudal tail too), and a single trimmed Idle loop. Body renders OPAQUE +
+// covers the caudal tail too), and a single trimmed loop (Swim1_norm — a ~2.5s in-place
+// swim cycle, zero root drift, so it undulates without traveling). Body renders OPAQUE +
 // single-sided; fins default to OPAQUE alpha-tested (clean silhouette, no blend bleed —
 // the koi's lesson) but can go translucent. All live-tunable at /zen?debug=1.
 export const DEFAULTS = {
@@ -120,7 +121,7 @@ export async function createAquarium(canvas, initial = {}) {
   const mixer = new THREE.AnimationMixer(ub);
   const actions = {};
   for (const clip of gltf.animations) actions[clip.name] = mixer.clipAction(clip);
-  const baseClip = mixer.clipAction(gltf.animations[0]);   // the trimmed Idle loop
+  const baseClip = mixer.clipAction(gltf.animations[0]);   // the trimmed swim loop
   baseClip.play();
   const baseDuration = baseClip.getClip().duration || 1;
   mixer.timeScale = reduced ? 0.6 : 1;   // calmer when reduced-motion