refactor: change meta_name to immutable reference and add debugging

Co-authored-by: aider (openai/andrew/openrouter/anthropic/claude-sonnet-4) <aider@aider.chat>

src/meta_plugin/digest.rs

@@ -69,7 +69,7 @@ impl MetaPlugin for DigestSha256MetaPlugin {
         self.hasher.update(data);
     }
 
-    fn meta_name(&mut self) -> String {
+    fn meta_name(&self) -> String {
         self.meta_name.clone()
     }
     
@@ -142,7 +142,7 @@ impl MetaPlugin for ReadTimeMetaPlugin {
         }
     }
 
-    fn meta_name(&mut self) -> String {
+    fn meta_name(&self) -> String {
         self.meta_name.clone()
     }
     
@@ -217,7 +217,7 @@ impl MetaPlugin for ReadRateMetaPlugin {
         self.bytes_read += data.len() as u64;
     }
 
-    fn meta_name(&mut self) -> String {
+    fn meta_name(&self) -> String {
         self.meta_name.clone()
     }