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/custom_logging.lux

@@ -0,0 +1,44 @@
+// Custom Logging with Effects
+//
+// This demonstrates how effects let you abstract side effects.
+// The same code can be run with different logging implementations.
+//
+// Expected output:
+// [INFO] Starting computation
+// [DEBUG] x = 10
+// [INFO] Processing
+// [DEBUG] result = 20
+// Final: 20
+
+effect Log {
+    fn info(msg: String): Unit
+    fn debug(msg: String): Unit
+}
+
+fn computation(): Int with {Log} = {
+    Log.info("Starting computation")
+    let x = 10
+    Log.debug("x = " + toString(x))
+    Log.info("Processing")
+    let result = x * 2
+    Log.debug("result = " + toString(result))
+    result
+}
+
+handler consoleLogger: Log {
+    fn info(msg) = {
+        Console.print("[INFO] " + msg)
+        resume(())
+    }
+    fn debug(msg) = {
+        Console.print("[DEBUG] " + msg)
+        resume(())
+    }
+}
+
+fn main(): Unit with {Console} = {
+    let result = run computation() with { Log = consoleLogger }
+    Console.print("Final: " + toString(result))
+}
+
+let output = run main() with {}