Add a comment explaining the lock --check error handling (#18595)

crates/uv/src/commands/project/lock.rs

@@ -253,6 +253,8 @@ pub(crate) async fn lock(
 
             Ok(ExitStatus::Success)
         }
+        // Lock mismatches from `--check`/`--locked` are expected validation failures.
+        // Handle them here so we return exit code 1 instead of bubbling up as an error (exit code 2).
         Err(err @ ProjectError::LockMismatch(..)) => {
             writeln!(printer.stderr(), "{}", err.to_string().bold())?;
             Ok(ExitStatus::Failure)