Reframe README as WinUI/XAML landing page; refresh CONTRIBUTING

README:
- Lead with "what Reactor is — and isn't" — every control is real WinUI
- Add "XAML and MVVM are not going anywhere" section validating the
  existing stack and describing how Reactor coexists with XAML/MVVM
- Add maturity table for the 17 investment areas (Preview/Draft/Early)
- Reorganize value prop by audience: Fundamentals / Frontier / Enterprise

CONTRIBUTING:
- Link to github.com/microsoft/microsoft-ui-reactor issues & discussions
- Reconcile test counts (2,200+ unit / 350+ selfhost / 46 E2E)
- Consolidate duplicated test-architecture table
- Fix numbering bug in "add a new element" guide
- Drop License section in favor of root LICENSE file

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: codemonkeychris