feat: improve error messages with context lines

- Show 2 lines of context before and after errors (dimmed)
- Fix guessing_game.lux to be non-interactive for testing
- Use binary search simulation to demonstrate game logic

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

examples/standard/guessing_game.lux

@@ -1,25 +1,44 @@
 // Number guessing game - demonstrates Random and Console effects
+//
+// Expected output:
+// Welcome to the Guessing Game!
+// Target number: 42
+// Simulating guesses...
+// Guess 50: Too high!
+// Guess 25: Too low!
+// Guess 37: Too low!
+// Guess 43: Too high!
+// Guess 40: Too low!
+// Guess 41: Too low!
+// Guess 42: Correct!
+// Found in 7 attempts!
 
-fn gameLoop(secret: Int, attempts: Int): Unit with {Console} = {
-    Console.print("Guess the number (1-100), attempt " + toString(attempts) + ":")
-    let guess = Console.readInt()
-    if guess == secret then
-        Console.print("Correct! You got it in " + toString(attempts) + " attempts!")
-    else if guess < secret then {
-        Console.print("Too low!")
-        gameLoop(secret, attempts + 1)
-    }
-    else {
-        Console.print("Too high!")
-        gameLoop(secret, attempts + 1)
-    }
+// Game logic - check a guess against the secret
+fn checkGuess(guess: Int, secret: Int): String =
+    if guess == secret then "Correct"
+    else if guess < secret then "Too low"
+    else "Too high"
+
+// Binary search simulation to find the number
+fn binarySearch(low: Int, high: Int, secret: Int, attempts: Int): Int with {Console} = {
+    let mid = (low + high) / 2
+    let result = checkGuess(mid, secret)
+    Console.print("Guess " + toString(mid) + ": " + result + "!")
+
+    if result == "Correct" then attempts
+    else if result == "Too low" then binarySearch(mid + 1, high, secret, attempts + 1)
+    else binarySearch(low, mid - 1, secret, attempts + 1)
 }
 
-fn main(): Unit with {Console, Random} = {
+fn main(): Unit with {Console} = {
     Console.print("Welcome to the Guessing Game!")
-    Console.print("I'm thinking of a number between 1 and 100...")
-    let secret = Random.int(1, 100)
-    gameLoop(secret, 1)
+    // Use a fixed "secret" for reproducible output
+    let secret = 42
+    Console.print("Target number: " + toString(secret))
+    Console.print("Simulating guesses...")
+
+    let attempts = binarySearch(1, 100, secret, 1)
+    Console.print("Found in " + toString(attempts) + " attempts!")
 }
 
 let output = run main() with {}