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/primes.lux

@@ -0,0 +1,29 @@
+// Prime number utilities
+
+fn isPrime(n: Int): Bool =
+    if n < 2 then false
+    else isPrimeHelper(n, 2)
+
+fn isPrimeHelper(n: Int, i: Int): Bool =
+    if i * i > n then true
+    else if n % i == 0 then false
+    else isPrimeHelper(n, i + 1)
+
+// Find first n primes
+fn findPrimes(count: Int): Unit with {Console} =
+    findPrimesHelper(2, count)
+
+fn findPrimesHelper(current: Int, remaining: Int): Unit with {Console} =
+    if remaining <= 0 then ()
+    else if isPrime(current) then {
+        Console.print(toString(current))
+        findPrimesHelper(current + 1, remaining - 1)
+    }
+    else findPrimesHelper(current + 1, remaining)
+
+fn main(): Unit with {Console} = {
+    Console.print("First 20 prime numbers:")
+    findPrimes(20)
+}
+
+let output = run main() with {}