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/guessing_game.lux

@@ -0,0 +1,25 @@
+// Number guessing game - demonstrates Random and Console effects
+
+fn gameLoop(secret: Int, attempts: Int): Unit with {Console} = {
+    Console.print("Guess the number (1-100), attempt " + toString(attempts) + ":")
+    let guess = Console.readInt()
+    if guess == secret then
+        Console.print("Correct! You got it in " + toString(attempts) + " attempts!")
+    else if guess < secret then {
+        Console.print("Too low!")
+        gameLoop(secret, attempts + 1)
+    }
+    else {
+        Console.print("Too high!")
+        gameLoop(secret, attempts + 1)
+    }
+}
+
+fn main(): Unit with {Console, Random} = {
+    Console.print("Welcome to the Guessing Game!")
+    Console.print("I'm thinking of a number between 1 and 100...")
+    let secret = Random.int(1, 100)
+    gameLoop(secret, 1)
+}
+
+let output = run main() with {}