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>

src/typechecker.rs

@@ -935,7 +935,7 @@ impl TypeChecker {
         }
 
         // Built-in effects are always available
-        let builtin_effects = ["Console", "Fail", "State", "Reader"];
+        let builtin_effects = ["Console", "Fail", "State", "Reader", "Random", "Time"];
         let is_builtin = builtin_effects.contains(&effect.name.as_str());
 
         // Track this effect for inference
@@ -1440,7 +1440,7 @@ impl TypeChecker {
 
         // Built-in effects are always available in run blocks (they have runtime implementations)
         let builtin_effects: EffectSet =
-            EffectSet::from_iter(["Console", "Fail", "State", "Reader"].iter().map(|s| s.to_string()));
+            EffectSet::from_iter(["Console", "Fail", "State", "Reader", "Random", "Time"].iter().map(|s| s.to_string()));
 
         // Extend current effects with handled ones and built-in effects
         let combined = self.current_effects.union(&handled_effects).union(&builtin_effects);