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/stdlib_demo.lux

@@ -0,0 +1,43 @@
+// Standard Library Demo
+// Demonstrates the built-in modules: List, String, Option, Math
+
+fn main(): Unit with {Console} = {
+    Console.print("=== List Operations ===")
+    let nums = [1, 2, 3, 4, 5]
+    Console.print("Original: " + toString(nums))
+    Console.print("Mapped (*2): " + toString(List.map(nums, fn(x: Int): Int => x * 2)))
+    Console.print("Filtered (even): " + toString(List.filter(nums, fn(x: Int): Bool => x % 2 == 0)))
+    Console.print("Sum (fold): " + toString(List.fold(nums, 0, fn(acc: Int, x: Int): Int => acc + x)))
+    Console.print("Length: " + toString(List.length(nums)))
+    Console.print("Reversed: " + toString(List.reverse(nums)))
+    Console.print("Range 1-5: " + toString(List.range(1, 6)))
+
+    Console.print("")
+    Console.print("=== String Operations ===")
+    let text = "  Hello, World!  "
+    Console.print("Original: \"" + text + "\"")
+    Console.print("Trimmed: \"" + String.trim(text) + "\"")
+    Console.print("Upper: " + String.toUpper(text))
+    Console.print("Lower: " + String.toLower(text))
+    Console.print("Contains 'World': " + toString(String.contains(text, "World")))
+    Console.print("Split by comma: " + toString(String.split("a,b,c", ",")))
+    Console.print("Join with dash: " + String.join(["x", "y", "z"], "-"))
+
+    Console.print("")
+    Console.print("=== Option Operations ===")
+    let some_val = Some(42)
+    let none_val: Option<Int> = None
+    Console.print("Some(42) mapped (*2): " + toString(Option.map(some_val, fn(x: Int): Int => x * 2)))
+    Console.print("None mapped: " + toString(Option.map(none_val, fn(x: Int): Int => x * 2)))
+    Console.print("Some(42) getOrElse(0): " + toString(Option.getOrElse(some_val, 0)))
+    Console.print("None getOrElse(0): " + toString(Option.getOrElse(none_val, 0)))
+
+    Console.print("")
+    Console.print("=== Math Operations ===")
+    Console.print("abs(-5): " + toString(Math.abs(-5)))
+    Console.print("min(3, 7): " + toString(Math.min(3, 7)))
+    Console.print("max(3, 7): " + toString(Math.max(3, 7)))
+    Console.print("pow(2, 10): " + toString(Math.pow(2, 10)))
+}
+
+let output = run main() with {}