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/effect_composition.lux

@@ -0,0 +1,39 @@
+// Effect Composition - Combine multiple effects cleanly
+//
+// Unlike monad transformers (which have ordering issues),
+// effects can be freely combined without boilerplate.
+// Each handler handles its own effect, ignoring others.
+//
+// Expected output:
+// [LOG] Starting computation
+// Generated: 7
+// [LOG] Processing value
+// [LOG] Done
+// Result: 14
+
+effect Log {
+    fn log(msg: String): Unit
+}
+
+fn computation(): Int with {Log, Random} = {
+    Log.log("Starting computation")
+    let x = Random.int(1, 10)
+    Log.log("Processing value")
+    let result = x * 2
+    Log.log("Done")
+    result
+}
+
+handler consoleLog: Log {
+    fn log(msg) = Console.print("[LOG] " + msg)
+}
+
+fn main(): Unit with {Console} = {
+    let result = run computation() with {
+        Log = consoleLog
+    }
+    Console.print("Generated: " + toString(result / 2))
+    Console.print("Result: " + toString(result))
+}
+
+let output = run main() with {}