feat: add HTTP client support

Add Http effect for making HTTP requests:
- Http.get(url) - GET request
- Http.post(url, body) - POST with string body
- Http.postJson(url, json) - POST with JSON body
- Http.put(url, body) - PUT request
- Http.delete(url) - DELETE request

Returns Result<HttpResponse, String> where HttpResponse contains
status code, body, and headers.

Includes reqwest dependency with blocking client, OpenSSL support
in flake.nix, and example at examples/http.lux demonstrating
API requests with JSON parsing.

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

flake.nix

@@ -49,6 +49,9 @@
           version = "0.1.0";
           src = ./.;
           cargoLock.lockFile = ./Cargo.lock;
+
+          nativeBuildInputs = [ pkgs.pkg-config ];
+          buildInputs = [ pkgs.openssl ];
         };
       }
     );