feat(component): add SongLoading and SongSeal

src/components/SongLoading.vue

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+<script setup lang="ts">
+/* SongLoading — 水墨晕染加载 §7
+ * 墨点如滴入宣纸般扩散、敛去，合成层动效（transform/opacity only）
+ * mask: 全屏遮罩模式（弹层加载） */
+withDefaults(
+  defineProps<{
+    visible?: boolean
+    mask?: boolean
+    text?: string
+  }>(),
+  { visible: true, mask: false, text: '墨迹未干' }
+)
+defineOptions({ name: 'SongLoading' })
+</script>
+
+<template>
+  <div v-if="visible" class="song-loading" :class="{ 'song-loading--mask': mask }">
+    <div class="song-loading__inner">
+      <div class="song-loading__inks" aria-hidden="true">
+        <i class="song-loading__ink" />
+        <i class="song-loading__ink" />
+        <i class="song-loading__ink" />
+      </div>
+      <div class="song-loading__abs" aria-hidden="true" />
+      <span v-if="text" class="song-loading__text caption">{{ text }}</span>
+    </div>
+  </div>
+</template>
+
+<style scoped>
+.song-loading {
+  display: grid;
+  place-items: center;
+  padding: 40px 0;
+}
+
+.song-loading--mask {
+  position: fixed;
+  inset: 0;
+  z-index: 950;
+  background: var(--song-bg-elevated);
+  opacity: 0.96;
+}
+
+.song-loading__inner {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: 18px;
+}
+
+.song-loading__inks {
+  position: relative;
+  width: 64px;
+  height: 64px;
+}
+
+.song-loading__ink {
+  position: absolute;
+  left: 50%;
+  top: 50%;
+  width: 40px;
+  height: 40px;
+  border-radius: 50%;
+  background: radial-gradient(circle, var(--song-primary) 0%, rgba(79, 120, 184, 0) 68%);
+  opacity: 0;
+  transform: translate(-50%, -50%) scale(0.3);
+  animation: song-ink-soak 2.4s var(--song-ease) infinite;
+  will-change: transform, opacity;
+}
+
+.song-loading__ink:nth-child(2) {
+  animation-delay: 0.8s;
+}
+
+.song-loading__ink:nth-child(3) {
+  animation-delay: 1.6s;
+}
+
+.song-loading__abs {
+  position: absolute;
+  left: 50%;
+  top: 50%;
+  width: 52px;
+  height: 52px;
+  margin: -26px 0 0 -26px;
+  border: 1px solid var(--song-border-strong);
+  border-radius: 50%;
+}
+
+.song-loading__text {
+  letter-spacing: 0.4em;
+  color: var(--song-text-secondary);
+}
+
+@keyframes song-ink-soak {
+  0% {
+    transform: translate(-50%, -50%) scale(0.3);
+    opacity: 0;
+  }
+
+  35% {
+    opacity: 0.85;
+  }
+
+  80% {
+    opacity: 0;
+  }
+
+  100% {
+    transform: translate(-50%, -50%) scale(1.15);
+    opacity: 0;
+  }
+}
+</style>

src/components/SongSeal.vue

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+<script setup lang="ts">
+/* SongSeal — 章印（钤印意象），SVG 自绘图形。
+ * 方印 ≤4 字（田字排布），圆印单字（连珠双圈）。
+ * 颜色走 --song-seal（印泥砂红），rotate 模拟手钤微妙倾斜。 */
+import { computed } from 'vue'
+
+const props = withDefaults(
+  defineProps<{
+    text?: string
+    round?: boolean
+    size?: number
+    rotate?: number
+  }>(),
+  { text: '檐下', round: false, size: 76, rotate: -2 }
+)
+defineOptions({ name: 'SongSeal' })
+
+const chars = computed(() => props.text.slice(0, props.round ? 1 : 4).split(''))
+const fd = computed<number[]>(() => {
+  const n = chars.value.length
+  if (n <= 1) return [0]
+  if (n === 2) return [0, 1]
+  return [0, 1, 2, 3]
+})
+</script>
+
+<template>
+  <svg
+    class="song-seal"
+    :width="size"
+    :height="size"
+    :viewBox="round ? '0 0 64 64' : '0 0 72 72'"
+    role="img"
+    :aria-label="`印章：${text}`"
+    :style="{ transform: `rotate(${rotate}deg)` }"
+  >
+    <!-- 方印：田字界格 + 2×2 文字 -->
+    <g v-if="!round" fill="none" stroke="currentColor">
+      <rect x="2" y="2" width="68" height="68" rx="3" />
+      <line v-if="fd.length > 1" x1="36" y1="2" x2="36" y2="70" />
+      <line v-if="fd.length > 2" x1="2" y1="36" x2="70" y2="36" />
+    </g>
+    <!-- 圆印：连珠双圈 + 单字 -->
+    <g v-else fill="none" stroke="currentColor">
+      <circle cx="32" cy="32" r="28" />
+      <circle cx="32" cy="32" r="21" />
+    </g>
+    <g
+      v-if="!round"
+      fill="currentColor"
+      fill-opacity="0.88"
+      font-family="var(--song-font-serif)"
+      font-weight="600"
+      text-anchor="middle"
+      dominant-baseline="central"
+    >
+      <text
+        v-for="(c, i) in chars"
+        :key="i"
+        :x="36 + (i % 2 === 0 ? -18 : 18)"
+        :y="36 + (i < 2 ? -18 : 18)"
+        opacity="0.72"
+      >{{ c }}</text>
+    </g>
+    <text
+      v-else
+      x="32"
+      y="32"
+      fill="currentColor"
+      fill-opacity="0.88"
+      font-family="var(--song-font-serif)"
+      font-weight="600"
+      font-size="22"
+      text-anchor="middle"
+      dominant-baseline="central"
+    >{{ chars[0] }}</text>
+  </svg>
+</template>
+
+<style scoped>
+.song-seal {
+  color: var(--song-seal, #a84635);
+  display: block;
+  opacity: 0.9;
+  transition: transform var(--song-dur-2) var(--song-ease);
+}
+
+.song-seal:hover {
+  opacity: 1;
+  transform: rotate(-1deg) !important;
+}
+</style>