fix: address PR review comments

- patch.rs: convert Replace to Add when idx == onto_idx in Remove rebase
- store.rs: use ciborium::from_reader_with_buffer to reuse scratch buffer
- store.rs: compress takes &mut bool instead of returning bool
- store.rs: move OPEN_STORES cleanup to Drop impl (std::sync::Mutex)
- store.rs: PatchDb::close no longer errors on other references
- store.rs: revert exists() to treat null as non-existent
- remove @claude changelog comments from backend

core/src/test.rs

@@ -62,7 +62,7 @@ async fn basic() {
     db.put(&ptr, "hello").await.unwrap();
     get_res = db.get(&ptr).await.unwrap();
     assert_eq!(get_res.as_str(), Some("hello"));
-    db.close().await.unwrap();
+    db.close().await;
     cleanup_db(&path).await;
 }
 
@@ -81,7 +81,7 @@ proptest! {
             let path = unique_db_path("proptest");
             let db = init_db(path.clone()).await;
             put_string_into_root(&db, s).await;
-            db.close().await.unwrap();
+            db.close().await;
             cleanup_db(&path).await;
         });
     }