feat: add Http effect to C backend

Implement HTTP client using POSIX sockets:
- Http.get(url) - GET request
- Http.post(url, body) - POST request
- Http.put(url, body) - PUT request
- Http.delete(url) - DELETE request

Features:
- Self-contained implementation (no libcurl dependency)
- URL parsing for host, port, and path
- HTTP/1.1 protocol with Connection: close
- Response body extraction

All Http operations use evidence passing for handler customization.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/C_BACKEND.md

@@ -301,11 +301,15 @@ Implemented C versions of:
 
 All effects use evidence passing for handler customization.
 
-### Phase 3: Http Effect
+### Phase 3: Http Effect ✅ COMPLETE
 
-Implement HTTP client:
+HTTP client using POSIX sockets:
 - `Http.get(url)` - GET request
 - `Http.post(url, body)` - POST request
+- `Http.put(url, body)` - PUT request
+- `Http.delete(url)` - DELETE request
+
+Self-contained implementation (no external dependencies like libcurl).
 
 ### Phase 5: JavaScript Backend