docs: add video component design spec

docs/superpowers/specs/2026-03-31-video-component-design.md

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+# Video Component Design
+
+## Summary
+
+A reusable `<Video>` MDX component for embedding videos in blog posts with sensible defaults and optional caption.
+
+## Component
+
+**File:** `components/video.tsx`
+
+**Props:**
+- `src: string` — video file path (required)
+- `caption?: string` — text displayed below video (optional)
+
+**Video defaults:** `controls loop muted playsInline width="100%"`
+
+**Styling:**
+- Container: `rounded-lg border border-white/30 bg-white/10 backdrop-blur-sm overflow-hidden shadow-md`
+- Caption: small, muted, italic, centered
+
+## Integration
+
+Register `Video` in `page.tsx` MDXRemote `components` prop so it's available in all MDX posts without per-post imports.
+
+## Usage in MDX
+
+```mdx
+<Video src="/posts/rts.mp4" />
+<Video src="/posts/rts.mp4" caption="My failed RTS attempt" />
+```