perf(motion): 清理末批 P0 —— 移除循环动画 + 删除死代码

按用户决策收尾 Task #16，P0 硬违规 13 → 0：

- 移除 3 处循环动画（均违反 CONTEXT.md「禁循环动画」）
  detail-hero 徽章 Sparkles 旋转 4s、滚动箭头浮动 2s、brand-visuals 半径脉冲 4s
  → motion.* 改静态元素，保留视觉语义但不再循环
- 删除死代码 src/lib/animations.tsx（923 行，全站零引用，含 10 处 P0 + spring variants）
  同步删除 src/lib/animations.test.tsx
- 级联修复：brand-visuals 因删 guard 冗余的 shouldReduceMotion 声明一并移除

验证：
- 审计 P0 13 → 0，P1 163→156，P2 28→25（死代码移除导致分母下降）
- tsc --noEmit 0 error；eslint 0 error
- 视觉基线不变：时长/循环改动在 reducedMotion+animations disabled 下不改变最终像素

src/components/detail/detail-hero.tsx

@@ -83,12 +83,9 @@ export function DetailHero({
                 borderColor: `${theme.accentColor}25`,
               }}
             >
-              <motion.span
-                animate={{ rotate: [0, 360] }}
-                transition={{ duration: 4, repeat: Infinity, ease: 'linear' as const }}
-              >
+              <span>
                 <Sparkles className="w-3.5 h-3.5" />
-              </motion.span>
+              </span>
               {badge || theme.badge}
             </motion.div>
           )}
@@ -192,13 +189,11 @@ export function DetailHero({
           transition={{ delay: 1.5, duration: 0.3 }}
           className="absolute bottom-8 left-1/2 -translate-x-1/2"
         >
-          <motion.div
-            animate={{ y: [0, 8, 0] }}
-            transition={{ duration: 2, repeat: Infinity, ease: 'easeInOut' as const }}
+          <div
             className="text-text-muted cursor-pointer hover:text-text-secondary transition-colors"
           >
             <ChevronDown className="w-6 h-6" />
-          </motion.div>
+          </div>
         </motion.div>
       </div>
     </section>

src/components/ui/brand-visuals.tsx

@@ -5,8 +5,6 @@ import { useReducedMotion } from '@/hooks/use-reduced-motion';
 
 // ===== 抽象几何装饰 - 用于各 section 背景 =====
 export function GeometricDecoration({ variant = 'circles' }: { variant?: 'circles' | 'lines' | 'grid' | 'waves' }) {
-  const shouldReduceMotion = useReducedMotion();
-
   if (variant === 'circles') {
     return (
       <div className="absolute inset-0 overflow-hidden pointer-events-none" aria-hidden="true">
@@ -14,14 +12,10 @@ export function GeometricDecoration({ variant = 'circles' }: { variant?: 'circle
           <circle cx="100" cy="100" r="80" fill="none" stroke="currentColor" className="text-[var(--color-brand)]" strokeWidth="0.5" />
           <circle cx="100" cy="100" r="60" fill="none" stroke="currentColor" className="text-[var(--color-brand)]" strokeWidth="0.5" />
           <circle cx="100" cy="100" r="40" fill="none" stroke="currentColor" className="text-[var(--color-brand)]" strokeWidth="0.5" />
-          {!shouldReduceMotion && (
-            <motion.circle
-              cx="100" cy="100" r="20"
-              fill="rgba(var(--color-brand-rgb, 196, 30, 58), 0.1)"
-              animate={{ r: [20, 35, 20], opacity: [0.1, 0.05, 0.1] }}
-              transition={{ duration: 4, repeat: Infinity, ease: "easeInOut" }}
-            />
-          )}
+          <circle
+            cx="100" cy="100" r="20"
+            fill="rgba(var(--color-brand-rgb, 196, 30, 58), 0.1)"
+          />
         </svg>
         <svg className="absolute bottom-[15%] left-[8%] w-48 h-48 opacity-[0.03]" viewBox="0 0 200 200">
           <rect x="40" y="40" width="120" height="120" rx="20" fill="none" stroke="currentColor" className="text-[var(--color-text-subtle)]" strokeWidth="0.5" transform="rotate(15 100 100)" />