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/standard/factorial.lux

@@ -0,0 +1,22 @@
+// Factorial - compute n!
+
+// Recursive version
+fn factorial(n: Int): Int =
+    if n <= 1 then 1
+    else n * factorial(n - 1)
+
+// Tail-recursive version (optimized)
+fn factorialTail(n: Int, acc: Int): Int =
+    if n <= 1 then acc
+    else factorialTail(n - 1, n * acc)
+
+fn factorial2(n: Int): Int = factorialTail(n, 1)
+
+fn main(): Unit with {Console} = {
+    Console.print("Factorial examples:")
+    Console.print("5! = " + toString(factorial(5)))
+    Console.print("10! = " + toString(factorial(10)))
+    Console.print("20! = " + toString(factorial2(20)))
+}
+
+let output = run main() with {}