feat: add comprehensive example programs

Standard examples (examples/standard/):
- hello_world: Basic effect usage
- fizzbuzz: Classic programming exercise
- factorial: Recursive and tail-recursive versions
- primes: Prime number generation
- guessing_game: Interactive Random + Console effects
- stdlib_demo: Demonstrates List, String, Option, Math modules

Showcase examples (examples/showcase/):
- ask_pattern: Resumable effects for config/environment
- custom_logging: Custom effect with handler
- early_return: Fail effect for clean error handling
- effect_composition: Combining multiple effects
- higher_order: Closures and function composition
- pattern_matching: ADTs and exhaustive matching

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

examples/showcase/higher_order.lux

@@ -0,0 +1,40 @@
+// Higher-Order Functions and Closures
+//
+// Functions are first-class values in Lux.
+// Closures capture their environment.
+//
+// Expected output:
+// Square of 5: 25
+// Cube of 3: 27
+// Add 10 to 5: 15
+// Add 10 to 20: 30
+// Composed: 15625 (cube(square(5)) = cube(25) = 15625)
+
+fn apply(f: fn(Int): Int, x: Int): Int = f(x)
+
+fn compose(f: fn(Int): Int, g: fn(Int): Int): fn(Int): Int =
+    fn(x: Int): Int => f(g(x))
+
+fn square(n: Int): Int = n * n
+
+fn cube(n: Int): Int = n * n * n
+
+fn makeAdder(n: Int): fn(Int): Int =
+    fn(x: Int): Int => x + n
+
+fn main(): Unit with {Console} = {
+    // Apply functions
+    Console.print("Square of 5: " + toString(apply(square, 5)))
+    Console.print("Cube of 3: " + toString(apply(cube, 3)))
+
+    // Closures
+    let add10 = makeAdder(10)
+    Console.print("Add 10 to 5: " + toString(add10(5)))
+    Console.print("Add 10 to 20: " + toString(add10(20)))
+
+    // Function composition
+    let squareThenCube = compose(cube, square)
+    Console.print("Composed: " + toString(squareThenCube(5)))
+}
+
+let output = run main() with {}