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/fizzbuzz.lux

@@ -0,0 +1,22 @@
+// FizzBuzz - print numbers 1-100, but:
+// - multiples of 3: print "Fizz"
+// - multiples of 5: print "Buzz"
+// - multiples of both: print "FizzBuzz"
+
+fn fizzbuzz(n: Int): String =
+    if n % 15 == 0 then "FizzBuzz"
+    else if n % 3 == 0 then "Fizz"
+    else if n % 5 == 0 then "Buzz"
+    else toString(n)
+
+fn printFizzbuzz(i: Int, max: Int): Unit with {Console} =
+    if i > max then ()
+    else {
+        Console.print(fizzbuzz(i))
+        printFizzbuzz(i + 1, max)
+    }
+
+fn main(): Unit with {Console} =
+    printFizzbuzz(1, 100)
+
+let output = run main() with {}