HTTP request/response: refinement hooks for valid requests and response status

[jaggederest]

Add refinement types for HTTP:

• ValidRequest — well-formed HTTP request (valid method, headers, URL structure)
• ValidStatus — response status code in valid range (100-599)
• Potentially SuccessStatus (2xx) vs ErrorStatus (4xx/5xx) refinements

Foundation for type-safe HTTP client/server APIs.