Rust test: pin the cargo-mutants survivors in the catalog lay-back

- The fixture gains an older entry listing the id ahead of the one that
  owns it, and the whole-store write carries the record under the
  registered type a Rust cache's pull gives it, not the entry's name.
  Kills the own-id lookup flip and the ||-to-&& flip in the entry filter.
- A store path that cannot be read (a directory) is an error, not an empty
  store (the NotFound guard had no test).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/cultcache-rs/src/lib.rs

@@ -4985,14 +4985,31 @@ mod tests {
             stored_at: "2026-09-30T00:00:00Z".into(),
             schema_id: Some("tests.other".into()),
         };
+        // An older entry that lists the id: the entry that owns the id is the one laid back.
+        let lister = PersistedSchemaCatalogEntry(
+            "sha256:older".into(),
+            "tests.note".into(),
+            "tests.note_v6".into(),
+            "sha256:older".into(),
+            String::new(),
+            vec!["sha256:older".into(), "sha256:csharp".into()],
+            Vec::new(),
+        );
         for whole_store in [false, true] {
             let temp = tempfile::tempdir()?;
             let path = temp.path().join("store.msgpack");
             let record = PersistedRecord("a".into(), "sha256:csharp".into(), "2026-09-30T00:00:00Z".into(), b"one".to_vec());
-            std::fs::write(&path, rmp_serde::to_vec(&PersistedStoreSnapshot("cultcache.store.v1".into(), vec![held.clone()], vec![record]))?)?;
+            std::fs::write(
+                &path,
+                rmp_serde::to_vec(&PersistedStoreSnapshot("cultcache.store.v1".into(), vec![lister.clone(), held.clone()], vec![record]))?,
+            )?;
             let mut store = SingleFileMessagePackBackingStore::new(&path);
             if whole_store {
+                // As a Rust cache holds it after a pull: under the registered type that reads the schema by name.
                 let mut entries = store.pull_all()?;
+                for entry in &mut entries {
+                    entry.r#type = "tests.rust_note".into();
+                }
                 entries.push(own.clone());
                 store.push_all(&entries, PushAllOptions::default())?;
             } else {
@@ -5007,6 +5024,27 @@ mod tests {
         Ok(())
     }
 
+    // Only a file that is not there is an empty store; any other failure to read it is an error, so a write never replaces a
+    // file it could not read.
+    #[test]
+    fn a_store_path_that_cannot_be_read_is_an_error_not_an_empty_store() -> Result<()> {
+        let temp = tempfile::tempdir()?;
+        let path = temp.path().join("store.msgpack");
+        std::fs::create_dir(&path)?;
+        let mut store = SingleFileMessagePackBackingStore::new(&path);
+        assert!(store.pull_all().is_err(), "a directory is not an empty store");
+        let own = CultCacheEnvelope {
+            key: "b".into(),
+            r#type: "tests.other".into(),
+            payload: b"two".to_vec(),
+            stored_at: "2026-09-30T00:00:00Z".into(),
+            schema_id: Some("tests.other".into()),
+        };
+        assert!(store.push(&own).is_err());
+        assert!(path.is_dir(), "the path is left as it was");
+        Ok(())
+    }
+
     fn store_file(path: &std::path::Path, catalog: &[(&str, &str)], records: &[(&str, &str, Vec<u8>)]) -> Result<()> {
         let snapshot = PersistedStoreSnapshot(
             "cultcache.store.v1".to_string(),