fix(fmt): ensure formatter always emits a trailing newline

README.md

@@ -4,7 +4,7 @@ A statically typed programming language.
 
 ```zy
 const std = import("std")
-std.debug.print("Hello World!")
+std.debug.print("Hello, 世界")
 ```
 
 ## License

examples/hello-world.zy

@@ -1,2 +1,2 @@
 const std = import("std")
-std.debug.print("Hello World!")
+std.debug.print("Hello, 世界")

src/commands/check.rs

@@ -165,8 +165,14 @@ fn fmt_diff(source: &str, formatted: &str) -> String {
             }
         }
     }
-    let trimmed_len = out.trim_end().len();
-    out.truncate(trimmed_len);
+    if out.is_empty() && source != formatted {
+        // Only difference is trailing newline — show as an added empty line
+        let ln = source.lines().count() + 1;
+        out.push_str(&crate::colors::green(&format!("{} | +", ln)));
+    } else {
+        let trimmed_len = out.trim_end().len();
+        out.truncate(trimmed_len);
+    }
     out
 }
 
@@ -196,7 +202,7 @@ fn check_and_report(
     let mut changed = false;
     if parse_errors.is_empty() {
         let formatted = crate::fmt::format_program(&ast, &blanks);
-        if formatted.trim() != source.trim() {
+        if formatted != source {
             if fix {
                 std::fs::write(path, &formatted)
                     .unwrap_or_else(|e| panic!("Failed to write '{}': {}", path, e));

src/fmt.rs

@@ -362,5 +362,8 @@ fn precedence(op: &BinOp) -> u8 {
 pub fn format_program(program: &crate::parser::Program, blanks: &[bool]) -> String {
     let mut f = Formatter::new();
     f.format_program(program, blanks);
+    if !f.buf.ends_with('\n') {
+        f.buf.push('\n');
+    }
     f.buf
 }

src/tests/fmt.rs

@@ -296,6 +296,27 @@ fn test_fmt_modulo() {
     idempotent(&out);
 }
 
+#[test]
+fn test_fmt_trailing_newline() {
+    let out = fmt("const x = 1");
+    assert!(
+        out.ends_with('\n'),
+        "expected trailing newline, got: {:?}",
+        out
+    );
+}
+
+#[test]
+fn test_fmt_trailing_newline_not_doubled() {
+    let out = fmt("const x = 1\n");
+    assert!(out.ends_with('\n'), "expected trailing newline");
+    assert!(
+        !out.ends_with("\n\n"),
+        "trailing newline doubled: {:?}",
+        out
+    );
+}
+
 #[test]
 fn test_fmt_fixtures_idempotent() {
     let fixtures = std::fs::read_dir("src/tests/fixtures")