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/json/README.md

@@ -0,0 +1,52 @@
+# json
+
+JSON parsing and serialization utilities for Lux.
+
+## Installation
+
+```bash
+lux pkg add json 1.0.0
+```
+
+## Usage
+
+```lux
+import json
+
+// Parse JSON string
+let data = json.parse('{"name": "Alice", "age": 30}')
+
+// Get fields
+let name = json.getString(data, "name")  // Some("Alice")
+let age = json.getInt(data, "age")       // Some(30)
+
+// Nested paths
+let city = json.getPath(data, "address.city")
+
+// Create JSON
+let obj = json.object([
+    ("name", JsonString("Bob")),
+    ("active", JsonBool(true))
+])
+
+// Stringify
+let str = json.stringify(obj)  // '{"name":"Bob","active":true}'
+```
+
+## API
+
+| Function | Description |
+|----------|-------------|
+| `parse(s)` | Parse JSON string |
+| `stringify(v)` | Convert to JSON string |
+| `getString(obj, key)` | Get string field |
+| `getInt(obj, key)` | Get integer field |
+| `getPath(obj, path)` | Get nested field by dot path |
+| `isNull(v)` | Check if value is null |
+| `object(pairs)` | Create JSON object |
+| `array(items)` | Create JSON array |
+| `prettyPrint(v)` | Pretty-print with indentation |
+
+## License
+
+MIT