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>

index.json

@@ -0,0 +1,28 @@
+{
+  "packages": [
+    {
+      "name": "json",
+      "description": "JSON parsing and serialization for Lux",
+      "latest_version": "1.0.0",
+      "versions": [
+        {"version": "1.0.0", "checksum": "", "published_at": "2025-02-15", "yanked": false}
+      ]
+    },
+    {
+      "name": "http-client",
+      "description": "HTTP client utilities for Lux",
+      "latest_version": "0.1.0",
+      "versions": [
+        {"version": "0.1.0", "checksum": "", "published_at": "2025-02-15", "yanked": false}
+      ]
+    },
+    {
+      "name": "testing",
+      "description": "Testing utilities and assertions for Lux",
+      "latest_version": "0.1.0",
+      "versions": [
+        {"version": "0.1.0", "checksum": "", "published_at": "2025-02-15", "yanked": false}
+      ]
+    }
+  ]
+}