The numbered section opener that drives every page's rhythm — mono index + descriptor, big Space Grotesk headline, optional lede.

```jsx
<SectionHeader index="03" eyebrow="Master plan"
  title="First, one. Then, everywhere."
  lede="Prove the pipeline. Patrol the water. Make orbit the intelligence layer for everything at sea." />
```

Set `onDark` on inverted sections, `align="center"` for closing CTAs. Pair the accent index with a muted eyebrow.
