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>

src/typechecker.rs

@@ -935,7 +935,7 @@ impl TypeChecker {
         }
 
         // Built-in effects are always available
-        let builtin_effects = ["Console", "Fail", "State", "Reader", "Random", "Time", "File", "Process"];
+        let builtin_effects = ["Console", "Fail", "State", "Reader", "Random", "Time", "File", "Process", "Http"];
         let is_builtin = builtin_effects.contains(&effect.name.as_str());
 
         // Track this effect for inference
@@ -1440,7 +1440,7 @@ impl TypeChecker {
 
         // Built-in effects are always available in run blocks (they have runtime implementations)
         let builtin_effects: EffectSet =
-            EffectSet::from_iter(["Console", "Fail", "State", "Reader", "Random", "Time", "File", "Process"].iter().map(|s| s.to_string()));
+            EffectSet::from_iter(["Console", "Fail", "State", "Reader", "Random", "Time", "File", "Process", "Http"].iter().map(|s| s.to_string()));
 
         // Extend current effects with handled ones and built-in effects
         let combined = self.current_effects.union(&handled_effects).union(&builtin_effects);