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/subscriber.rs

@@ -93,10 +93,6 @@ impl DbWatch {
         self.seen = true;
         Ok(self.state.clone())
     }
-    // @claude fix #9: Previously applied only one revision per poll, emitting
-    // intermediate states that may never have been a consistent committed state.
-    // Now drains all queued revisions after the first wake, matching sync()
-    // behavior so the caller always sees a fully caught-up snapshot.
     pub fn poll_changed(&mut self, cx: &mut std::task::Context<'_>) -> Poll<Result<(), Error>> {
         if !self.seen {
             self.seen = true;