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/testing/README.md

@@ -0,0 +1,56 @@
+# testing
+
+Testing utilities and assertions for Lux.
+
+## Installation
+
+```bash
+lux pkg add testing 0.1.0
+```
+
+## Usage
+
+```lux
+import testing
+
+fn testMyFunction(): Unit with {Console, Test} = {
+    testing.describe("MyFunction", fn() => {
+        testing.it("should return correct value", fn() => {
+            let result = myFunction(5)
+            testing.assertEqual(result, 10, "5 * 2 should be 10")
+        })
+
+        testing.it("should handle edge cases", fn() => {
+            let result = myFunction(0)
+            testing.assertEqual(result, 0, "0 * 2 should be 0")
+        })
+    })
+}
+```
+
+## Assertions
+
+| Function | Description |
+|----------|-------------|
+| `assert(cond, msg)` | Assert condition is true |
+| `assertEqual(a, b, msg)` | Assert two values are equal |
+| `assertSome(opt, msg)` | Assert Option is Some |
+| `assertNone(opt, msg)` | Assert Option is None |
+| `assertOk(result, msg)` | Assert Result is Ok |
+| `assertErr(result, msg)` | Assert Result is Err |
+| `assertEmpty(list, msg)` | Assert list is empty |
+| `assertLength(list, n, msg)` | Assert list has length n |
+| `assertContains(s, sub, msg)` | Assert string contains substring |
+| `assertStartsWith(s, pre, msg)` | Assert string starts with prefix |
+| `assertEndsWith(s, suf, msg)` | Assert string ends with suffix |
+
+## Test Structure
+
+| Function | Description |
+|----------|-------------|
+| `describe(name, tests)` | Group related tests |
+| `it(desc, test)` | Define individual test |
+
+## License
+
+MIT