feat: implement built-in State, Reader, and Fail effects

Add runtime support for the State, Reader, and Fail effects that
were already defined in the type system. These effects can now be
used in effectful code blocks.

Changes:
- Add builtin_state and builtin_reader fields to Interpreter
- Implement State.get and State.put in handle_builtin_effect
- Implement Reader.ask in handle_builtin_effect
- Add Reader effect definition to types.rs
- Add Reader to built-in effects list in typechecker
- Add set_state/get_state/set_reader/get_reader methods
- Add 6 new tests for built-in effects
- Add examples/builtin_effects.lux demonstrating usage

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

src/types.rs

@@ -788,6 +788,20 @@ impl TypeEnv {
             },
         );
 
+        // Add Reader effect
+        env.effects.insert(
+            "Reader".to_string(),
+            EffectDef {
+                name: "Reader".to_string(),
+                type_params: vec!["R".to_string()],
+                operations: vec![EffectOpDef {
+                    name: "ask".to_string(),
+                    params: Vec::new(),
+                    return_type: Type::Var(0), // R
+                }],
+            },
+        );
+
         // Add Some and Ok, Err constructors
         // Some : fn(a) -> Option<a>
         let a = Type::var();