fix: drop unneeded mut on migrate-input write closure

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01PXSsCppeDFmSRRM5NAug5a

 src/migrate_input.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/migrate_input.rs b/src/migrate_input.rs
index 9d8d9c8..3e41840 100644
--- a/src/migrate_input.rs
+++ b/src/migrate_input.rs
@@ -230,7 +230,7 @@ pub fn run() {
     if let Some(bak) = backup(&input_path) {
         println!("backed up {} -> {}", input_path.display(), bak.display());
     }
-    let mut write = |domain: &str, entries: &[BindingEntry], added: usize| {
+    let write = |domain: &str, entries: &[BindingEntry], added: usize| {
         if added == 0 {
             return;
         }