添加 hallmark 技能

skills/hallmark/references/components/s2-hanging.md

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+### S2 · Hanging
+Heading floats above the section in negative space; no border, no rule.
+*Use when:* the content has a quiet, room-to-breathe energy.
+*Don't confuse with:* S3 Sticky-pinned (which moves with scroll).
+
+```html
+<header class="head-hang">
+  <h2>…</h2>
+</header>
+```
+```css
+.head-hang { padding-block: var(--space-3xl) var(--space-xl); }
+```