feat: initialize Lux package registry

- Add registry README with API documentation
- Add initial packages: json, http-client, testing
- Add package index.json for registry server

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

packages/http-client/README.md

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+# http-client
+
+HTTP client utilities for Lux.
+
+## Installation
+
+```bash
+lux pkg add http-client 0.1.0
+```
+
+## Usage
+
+```lux
+import http-client as http
+
+fn main(): Unit with {Console, Http} = {
+    // Simple GET request
+    let body = http.get("https://api.example.com/data")
+    Console.print(body)
+
+    // GET and parse JSON
+    let data = http.getJson("https://api.example.com/users")
+    Console.print("Got " ++ toString(List.length(data)) ++ " users")
+
+    // POST with JSON body
+    let response = http.postJson(
+        "https://api.example.com/users",
+        Json.object([
+            ("name", JsonString("Alice")),
+            ("email", JsonString("alice@example.com"))
+        ])
+    )
+
+    // GET with query parameters
+    let results = http.getWithParams(
+        "https://api.example.com/search",
+        [("q", "lux programming"), ("limit", "10")]
+    )
+}
+```
+
+## API
+
+| Function | Description |
+|----------|-------------|
+| `get(url)` | GET request, return body |
+| `getJson(url)` | GET and parse as JSON |
+| `postJson(url, data)` | POST with JSON body |
+| `putJson(url, data)` | PUT with JSON body |
+| `delete(url)` | DELETE request |
+| `postJsonGetJson(url, data)` | POST JSON, parse response |
+| `getWithParams(url, params)` | GET with query parameters |
+| `urlEncode(s)` | URL-encode a string |
+| `buildQueryString(params)` | Build query string from pairs |
+
+## License
+
+MIT

packages/http-client/lib.lux

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+// HTTP client utilities for Lux
+//
+// Provides convenient wrappers around the Http effect.
+
+// HTTP response type
+type Response = {
+    status: Int,
+    body: String,
+    headers: List<(String, String)>
+}
+
+// Make a GET request and return the body
+pub fn get(url: String): String with Http = {
+    Http.get(url)
+}
+
+// Make a POST request with JSON body
+pub fn postJson(url: String, data: JsonValue): String with Http = {
+    Http.post(url, Json.stringify(data))
+}
+
+// Make a PUT request with JSON body
+pub fn putJson(url: String, data: JsonValue): String with Http = {
+    Http.put(url, Json.stringify(data))
+}
+
+// Make a DELETE request
+pub fn delete(url: String): String with Http = {
+    Http.delete(url)
+}
+
+// Fetch JSON and parse it
+pub fn getJson(url: String): JsonValue with Http = {
+    let body = Http.get(url)
+    Json.parse(body)
+}
+
+// Post JSON and parse response as JSON
+pub fn postJsonGetJson(url: String, data: JsonValue): JsonValue with Http = {
+    let body = Http.post(url, Json.stringify(data))
+    Json.parse(body)
+}
+
+// URL encode a string
+pub fn urlEncode(s: String): String = {
+    // Simple URL encoding
+    String.replace(
+        String.replace(
+            String.replace(s, " ", "%20"),
+            "&", "%26"
+        ),
+        "=", "%3D"
+    )
+}
+
+// Build query string from parameters
+pub fn buildQueryString(params: List<(String, String)>): String = {
+    let encoded = List.map(params, fn(pair) => {
+        urlEncode(pair.0) ++ "=" ++ urlEncode(pair.1)
+    })
+    String.join(encoded, "&")
+}
+
+// Make a GET request with query parameters
+pub fn getWithParams(baseUrl: String, params: List<(String, String)>): String with Http = {
+    let qs = buildQueryString(params)
+    let url = if String.contains(baseUrl, "?") then
+        baseUrl ++ "&" ++ qs
+    else
+        baseUrl ++ "?" ++ qs
+    Http.get(url)
+}

packages/http-client/lux.toml

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+[project]
+name = "http-client"
+version = "0.1.0"
+description = "HTTP client utilities for Lux"
+authors = ["Lux Core Team"]
+license = "MIT"
+
+[dependencies]