Add promotional introduction to README

[gjbkz]

Rewrites the top of README.md to lead with a compelling value proposition:

• Explains the "define once, validate everywhere" core benefit up front
• Lists key selling points (built-in validators, recursive types, caching, etc.)
• Shows a minimal but complete code snippet immediately

https://claude.ai/code/session_01AfxKyjrKBq9cT5698HAZxF

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.86%. Comparing base (c16273b) to head (43f8799).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #423   +/-   ##
=======================================
  Coverage   96.86%   96.86%           
=======================================
  Files          45       45           
  Lines        2104     2104           
  Branches      209      209           
=======================================
  Hits         2038     2038           
  Misses         66       66           

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