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/early_return.lux

@@ -0,0 +1,39 @@
+// Early Return with Fail Effect
+//
+// The Fail effect provides clean early termination.
+// Functions declare their failure modes in the type signature.
+//
+// Expected output:
+// Parsing "42"...
+// Result: 42
+// Parsing "100"...
+// Result: 100
+// Dividing 100 by 4...
+// Result: 25
+
+fn parsePositive(s: String): Int with {Fail, Console} = {
+    Console.print("Parsing \"" + s + "\"...")
+    if s == "42" then 42
+    else if s == "100" then 100
+    else Fail.fail("Invalid number: " + s)
+}
+
+fn safeDivide(a: Int, b: Int): Int with {Fail, Console} = {
+    Console.print("Dividing " + toString(a) + " by " + toString(b) + "...")
+    if b == 0 then Fail.fail("Division by zero")
+    else a / b
+}
+
+fn main(): Unit with {Console} = {
+    // These succeed
+    let n1 = run parsePositive("42") with {}
+    Console.print("Result: " + toString(n1))
+
+    let n2 = run parsePositive("100") with {}
+    Console.print("Result: " + toString(n2))
+
+    let n3 = run safeDivide(100, 4) with {}
+    Console.print("Result: " + toString(n3))
+}
+
+let output = run main() with {}