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>

examples/http.lux

@@ -0,0 +1,91 @@
+// HTTP example - demonstrates the Http effect
+//
+// This script makes HTTP requests and parses JSON responses
+
+fn main(): Unit with {Console, Http} = {
+    Console.print("=== Lux HTTP Example ===")
+    Console.print("")
+
+    // Make a GET request to a public API
+    Console.print("Fetching data from httpbin.org...")
+    Console.print("")
+
+    match Http.get("https://httpbin.org/get") {
+        Ok(response) => {
+            Console.print("GET request successful!")
+            Console.print("  Status: " + toString(response.status))
+            Console.print("  Body length: " + toString(String.length(response.body)) + " bytes")
+            Console.print("")
+
+            // Parse the JSON response
+            match Json.parse(response.body) {
+                Ok(json) => {
+                    Console.print("Parsed JSON response:")
+                    match Json.get(json, "origin") {
+                        Some(origin) => match Json.asString(origin) {
+                            Some(ip) => Console.print("  Your IP: " + ip),
+                            None => Console.print("  origin: (not a string)")
+                        },
+                        None => Console.print("  origin: (not found)")
+                    }
+                    match Json.get(json, "url") {
+                        Some(url) => match Json.asString(url) {
+                            Some(u) => Console.print("  URL: " + u),
+                            None => Console.print("  url: (not a string)")
+                        },
+                        None => Console.print("  url: (not found)")
+                    }
+                },
+                Err(e) => Console.print("JSON parse error: " + e)
+            }
+        },
+        Err(e) => Console.print("GET request failed: " + e)
+    }
+
+    Console.print("")
+    Console.print("--- POST Request ---")
+    Console.print("")
+
+    // Make a POST request with JSON body
+    let requestBody = Json.object([("message", Json.string("Hello from Lux!")), ("version", Json.int(1))])
+    Console.print("Sending POST with JSON body...")
+    Console.print("  Body: " + Json.stringify(requestBody))
+    Console.print("")
+
+    match Http.postJson("https://httpbin.org/post", requestBody) {
+        Ok(response) => {
+            Console.print("POST request successful!")
+            Console.print("  Status: " + toString(response.status))
+
+            // Parse and extract what we sent
+            match Json.parse(response.body) {
+                Ok(json) => match Json.get(json, "json") {
+                    Some(sentJson) => {
+                        Console.print("  Server received:")
+                        Console.print("    " + Json.stringify(sentJson))
+                    },
+                    None => Console.print("  (no json field in response)")
+                },
+                Err(e) => Console.print("JSON parse error: " + e)
+            }
+        },
+        Err(e) => Console.print("POST request failed: " + e)
+    }
+
+    Console.print("")
+    Console.print("--- Headers ---")
+    Console.print("")
+
+    // Show response headers
+    match Http.get("https://httpbin.org/headers") {
+        Ok(response) => {
+            Console.print("Response headers (first 5):")
+            let count = 0
+            // Note: Can't easily iterate with effects in callbacks, so just show count
+            Console.print("  Total headers: " + toString(List.length(response.headers)))
+        },
+        Err(e) => Console.print("Request failed: " + e)
+    }
+}
+
+let result = run main() with {}