perf(app): make SiteHeader a server component with a client island

[jdjioe5-cpu]

Summary

Closes #41

Cuts first-load JavaScript on the root layout by refactoring `SiteHeader` into a Server Component with a dedicated, minimal client island:

1. Server Component Transformation: Converted `components/nav/SiteHeader.tsx` into a Server Component (removed `"use client"`), rendering layout structure, brand lock icon with SVG mask definitions, and static external anchors (Docs and GitHub) directly as server HTML stream.
2. Client Island Extraction: Created `components/nav/NavLinks.tsx` containing only the dynamic interactive hooks:
   • `DesktopNavLinks`: Uses `usePathname` for active link `aria-current="page"` highlighting.
   • `MobileMenu`: Handles mobile disclosure hamburger button, `open` state, route change auto-collapse, and `Escape` key dismissal with focus restoration to `toggleRef`.
3. Accessibility & Behavior Preserved:
   • `motion-reduce:transition-none` transitions retained on active links and hamburger animation.
   • All ARIA attributes (`aria-expanded`, `aria-controls`, `aria-label`, `aria-current`) intact.
   • Full keyboard navigation and escape key dismissal verified.
4. Performance Measurement: Documented before/after analysis in `docs/PERFORMANCE.md`.
5. Roadmap Tracking: Updated `ROADMAP.md` to set `M3-18` to IN PROGRESS, recording Changelog and Last updated.

Hard rules touched

• AGENTS.md §8.3 (mobile-first, fast load on slow connections, WCAG AA / reduced-motion preserved)
• AGENTS.md §12 (ROADMAP.md updated with accurate changelog)

Verify output

```bash
$ pnpm lint
Checked 84 files in 21ms. No fixes applied.

$ pnpm typecheck
tsc --noEmit (exited with 0)

$ pnpm test
Test Files 15 passed (15)
Tests 168 passed (168)
Duration 739ms

$ pnpm build
▲ Next.js 16.3.8 (Turbopack)
✓ Compiled successfully in 443ms
✓ Generating static pages using 14 workers (6/6) in 424ms
```