feat: implement documentation comments

Add support for doc comments (/// syntax) that can be attached to
declarations for documentation purposes. The implementation:

- Adds DocComment token kind to lexer
- Recognizes /// as doc comment syntax (distinct from // regular comments)
- Parses consecutive doc comments and combines them into a single string
- Adds doc field to FunctionDecl, TypeDecl, LetDecl, EffectDecl, TraitDecl
- Passes doc comments through parser to declarations
- Multiple consecutive doc comment lines are joined with newlines

This enables documentation extraction and could be used for generating
API docs, IDE hover information, and REPL help.

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

src/main.rs

@@ -1419,5 +1419,42 @@ c")"#;
             let result = eval(source);
             assert!(result.is_ok(), "Expected success with explicit effects but got: {:?}", result);
         }
+
+        #[test]
+        fn test_doc_comments_on_function() {
+            // Test that doc comments are parsed and attached to functions
+            let source = r#"
+                /// Adds two numbers together.
+                /// Returns the sum.
+                fn add(a: Int, b: Int): Int = a + b
+                let result = add(1, 2)
+            "#;
+            assert_eq!(eval(source).unwrap(), "3");
+        }
+
+        #[test]
+        fn test_doc_comments_on_type() {
+            // Test that doc comments are parsed and attached to types
+            let source = r#"
+                /// A point in 2D space.
+                type Point { x: Int, y: Int }
+                let p = { x: 1, y: 2 }
+                let result = p.x + p.y
+            "#;
+            assert_eq!(eval(source).unwrap(), "3");
+        }
+
+        #[test]
+        fn test_doc_comments_multiline() {
+            // Test that multiple doc comment lines are combined
+            let source = r#"
+                /// First line of documentation.
+                /// Second line of documentation.
+                /// Third line of documentation.
+                fn documented(): Int = 42
+                let result = documented()
+            "#;
+            assert_eq!(eval(source).unwrap(), "42");
+        }
     }
 }