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/pattern_matching.lux

@@ -0,0 +1,55 @@
+// Algebraic Data Types and Pattern Matching
+//
+// Lux has powerful ADTs with exhaustive pattern matching.
+// The type system ensures all cases are handled.
+//
+// Expected output:
+// Evaluating: (2 + 3)
+// Result: 5
+// Evaluating: ((1 + 2) * (3 + 4))
+// Result: 21
+// Evaluating: (10 - (2 * 3))
+// Result: 4
+
+type Expr =
+    | Num(Int)
+    | Add(Expr, Expr)
+    | Sub(Expr, Expr)
+    | Mul(Expr, Expr)
+
+fn eval(e: Expr): Int =
+    match e {
+        Num(n) => n,
+        Add(a, b) => eval(a) + eval(b),
+        Sub(a, b) => eval(a) - eval(b),
+        Mul(a, b) => eval(a) * eval(b)
+    }
+
+fn showExpr(e: Expr): String =
+    match e {
+        Num(n) => toString(n),
+        Add(a, b) => "(" + showExpr(a) + " + " + showExpr(b) + ")",
+        Sub(a, b) => "(" + showExpr(a) + " - " + showExpr(b) + ")",
+        Mul(a, b) => "(" + showExpr(a) + " * " + showExpr(b) + ")"
+    }
+
+fn evalAndPrint(e: Expr): Unit with {Console} = {
+    Console.print("Evaluating: " + showExpr(e))
+    Console.print("Result: " + toString(eval(e)))
+}
+
+fn main(): Unit with {Console} = {
+    // (2 + 3)
+    let e1 = Add(Num(2), Num(3))
+    evalAndPrint(e1)
+
+    // ((1 + 2) * (3 + 4))
+    let e2 = Mul(Add(Num(1), Num(2)), Add(Num(3), Num(4)))
+    evalAndPrint(e2)
+
+    // (10 - (2 * 3))
+    let e3 = Sub(Num(10), Mul(Num(2), Num(3)))
+    evalAndPrint(e3)
+}
+
+let output = run main() with {}