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/ask_pattern.lux

@@ -0,0 +1,39 @@
+// The "Ask" Pattern - Resumable Effects
+//
+// Unlike exceptions which unwind the stack, effect handlers can
+// RESUME with a value. This enables "ask the environment" patterns.
+//
+// Expected output:
+// Need config: api_url
+// Got: https://api.example.com
+// Need config: timeout
+// Got: 30
+// Configured with url=https://api.example.com, timeout=30
+
+effect Config {
+    fn get(key: String): String
+}
+
+fn configure(): String with {Config, Console} = {
+    Console.print("Need config: api_url")
+    let url = Config.get("api_url")
+    Console.print("Got: " + url)
+    Console.print("Need config: timeout")
+    let timeout = Config.get("timeout")
+    Console.print("Got: " + timeout)
+    "Configured with url=" + url + ", timeout=" + timeout
+}
+
+handler envConfig: Config {
+    fn get(key) =
+        if key == "api_url" then resume("https://api.example.com")
+        else if key == "timeout" then resume("30")
+        else resume("unknown")
+}
+
+fn main(): Unit with {Console} = {
+    let result = run configure() with { Config = envConfig }
+    Console.print(result)
+}
+
+let output = run main() with {}