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/hello_world.lux

@@ -0,0 +1,7 @@
+// The classic first program
+// Expected output: Hello, World!
+
+fn main(): Unit with {Console} =
+    Console.print("Hello, World!")
+
+let output = run main() with {}