generate: drop a needless mut

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L4jrCgLiKKHAEFuZUJjckH

src/main.rs

@@ -301,7 +301,7 @@ fn parse_files(text: &str) -> anyhow::Result<BTreeMap<String, String>> {
     // opening, and a new FILE closes whatever was open.
     let mut files = BTreeMap::new();
     let mut current: Option<(String, String)> = None;
-    let mut close = |current: &mut Option<(String, String)>, files: &mut BTreeMap<String, String>| {
+    let close = |current: &mut Option<(String, String)>, files: &mut BTreeMap<String, String>| {
         if let Some((path, body)) = current.take() {
             files.insert(path, body);
         }