feat: implement Random and Time built-in effects

Add Random and Time effects for random number generation and
time-based operations. These effects can be used in any
effectful code block.

Random effect operations:
- Random.int(min, max) - random integer in range [min, max]
- Random.float() - random float in range [0.0, 1.0)
- Random.bool() - random boolean

Time effect operations:
- Time.now() - current Unix timestamp in milliseconds
- Time.sleep(ms) - sleep for specified milliseconds

Changes:
- Add rand crate dependency
- Add Random and Time effect definitions to types.rs
- Add effects to built-in effects list in typechecker
- Implement effect handlers in interpreter
- Add 4 new tests for Random and Time effects
- Add examples/random.lux demonstrating usage

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

examples/random.lux

@@ -0,0 +1,41 @@
+// Demonstrating Random and Time effects in Lux
+//
+// Expected output (values will vary):
+// Rolling dice...
+// Die 1: <random 1-6>
+// Die 2: <random 1-6>
+// Die 3: <random 1-6>
+// Coin flip: <true/false>
+// Random float: <0.0-1.0>
+// Current time: <timestamp>
+
+// Roll a single die (1-6)
+fn rollDie(): Int with {Random} = Random.int(1, 6)
+
+// Roll multiple dice and print results
+fn rollDice(count: Int): Unit with {Random, Console} = {
+    if count > 0 then {
+        let value = rollDie()
+        Console.print("Die " + toString(4 - count) + ": " + toString(value))
+        rollDice(count - 1)
+    } else {
+        ()
+    }
+}
+
+// Main function demonstrating random effects
+fn main(): Unit with {Random, Console, Time} = {
+    Console.print("Rolling dice...")
+    rollDice(3)
+
+    let coin = Random.bool()
+    Console.print("Coin flip: " + toString(coin))
+
+    let f = Random.float()
+    Console.print("Random float: " + toString(f))
+
+    let now = Time.now()
+    Console.print("Current time: " + toString(now))
+}
+
+let output = run main() with {}