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/lib.lux

@@ -0,0 +1,71 @@
+// Testing utilities for Lux
+//
+// Provides assertion functions and test helpers.
+
+// Assert that a condition is true
+pub fn assert(condition: Bool, message: String): Unit with Test = {
+    Test.assertTrue(condition, message)
+}
+
+// Assert two values are equal
+pub fn assertEqual<T>(actual: T, expected: T, message: String): Unit with Test = {
+    Test.assertEqual(actual, expected, message)
+}
+
+// Assert a value is Some
+pub fn assertSome<T>(opt: Option<T>, message: String): Unit with Test = {
+    Test.assertTrue(Option.isSome(opt), message)
+}
+
+// Assert a value is None
+pub fn assertNone<T>(opt: Option<T>, message: String): Unit with Test = {
+    Test.assertTrue(Option.isNone(opt), message)
+}
+
+// Assert a Result is Ok
+pub fn assertOk<T, E>(result: Result<T, E>, message: String): Unit with Test = {
+    Test.assertTrue(Result.isOk(result), message)
+}
+
+// Assert a Result is Err
+pub fn assertErr<T, E>(result: Result<T, E>, message: String): Unit with Test = {
+    Test.assertTrue(Result.isErr(result), message)
+}
+
+// Assert a list is empty
+pub fn assertEmpty<T>(list: List<T>, message: String): Unit with Test = {
+    Test.assertTrue(List.isEmpty(list), message)
+}
+
+// Assert a list has specific length
+pub fn assertLength<T>(list: List<T>, expected: Int, message: String): Unit with Test = {
+    Test.assertEqual(List.length(list), expected, message)
+}
+
+// Assert a string contains a substring
+pub fn assertContains(haystack: String, needle: String, message: String): Unit with Test = {
+    Test.assertTrue(String.contains(haystack, needle), message)
+}
+
+// Assert a string starts with a prefix
+pub fn assertStartsWith(s: String, prefix: String, message: String): Unit with Test = {
+    Test.assertTrue(String.startsWith(s, prefix), message)
+}
+
+// Assert a string ends with a suffix
+pub fn assertEndsWith(s: String, suffix: String, message: String): Unit with Test = {
+    Test.assertTrue(String.endsWith(s, suffix), message)
+}
+
+// Run a test suite
+pub fn describe(name: String, tests: fn(): Unit with Test): Unit with {Console, Test} = {
+    Console.print("Running: " ++ name)
+    tests()
+    Console.print("Passed: " ++ name)
+}
+
+// Individual test case
+pub fn it(description: String, test: fn(): Unit with Test): Unit with {Console, Test} = {
+    Console.print("  - " ++ description)
+    test()
+}