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/types.rs

@@ -802,6 +802,56 @@ impl TypeEnv {
             },
         );
 
+        // Add Random effect
+        env.effects.insert(
+            "Random".to_string(),
+            EffectDef {
+                name: "Random".to_string(),
+                type_params: Vec::new(),
+                operations: vec![
+                    EffectOpDef {
+                        name: "int".to_string(),
+                        params: vec![
+                            ("min".to_string(), Type::Int),
+                            ("max".to_string(), Type::Int),
+                        ],
+                        return_type: Type::Int,
+                    },
+                    EffectOpDef {
+                        name: "float".to_string(),
+                        params: Vec::new(),
+                        return_type: Type::Float,
+                    },
+                    EffectOpDef {
+                        name: "bool".to_string(),
+                        params: Vec::new(),
+                        return_type: Type::Bool,
+                    },
+                ],
+            },
+        );
+
+        // Add Time effect
+        env.effects.insert(
+            "Time".to_string(),
+            EffectDef {
+                name: "Time".to_string(),
+                type_params: Vec::new(),
+                operations: vec![
+                    EffectOpDef {
+                        name: "now".to_string(),
+                        params: Vec::new(),
+                        return_type: Type::Int, // Unix timestamp in milliseconds
+                    },
+                    EffectOpDef {
+                        name: "sleep".to_string(),
+                        params: vec![("ms".to_string(), Type::Int)],
+                        return_type: Type::Unit,
+                    },
+                ],
+            },
+        );
+
         // Add Some and Ok, Err constructors
         // Some : fn(a) -> Option<a>
         let a = Type::var();