feat(storage): add offline data validation and strengthen storage/index integrity checks

rust/crates/rustdb-index/src/engine.rs

@@ -153,6 +153,55 @@ impl IndexEngine {
         self.indexes.contains_key(name)
     }
 
+    /// Check unique constraints for a document without modifying the index.
+    /// Returns Ok(()) if no conflict, Err(DuplicateKey) if a unique constraint
+    /// would be violated. This is a read-only check (immutable &self).
+    pub fn check_unique_constraints(&self, doc: &Document) -> Result<(), IndexError> {
+        for idx in self.indexes.values() {
+            if idx.unique {
+                let key_bytes = Self::extract_key_bytes(doc, &idx.key, idx.sparse);
+                if let Some(ref kb) = key_bytes {
+                    if let Some(existing_ids) = idx.hash.get(kb) {
+                        if !existing_ids.is_empty() {
+                            return Err(IndexError::DuplicateKey {
+                                index: idx.name.clone(),
+                                key: format!("{:?}", kb),
+                            });
+                        }
+                    }
+                }
+            }
+        }
+        Ok(())
+    }
+
+    /// Check unique constraints for an update, excluding the document being updated.
+    /// Returns Ok(()) if no conflict. This is a read-only check (immutable &self).
+    pub fn check_unique_constraints_for_update(
+        &self,
+        old_doc: &Document,
+        new_doc: &Document,
+    ) -> Result<(), IndexError> {
+        let doc_id = Self::extract_id(old_doc);
+        for idx in self.indexes.values() {
+            if idx.unique {
+                let new_key_bytes = Self::extract_key_bytes(new_doc, &idx.key, idx.sparse);
+                if let Some(ref kb) = new_key_bytes {
+                    if let Some(existing_ids) = idx.hash.get(kb) {
+                        let has_conflict = existing_ids.iter().any(|id| *id != doc_id);
+                        if has_conflict {
+                            return Err(IndexError::DuplicateKey {
+                                index: idx.name.clone(),
+                                key: format!("{:?}", kb),
+                            });
+                        }
+                    }
+                }
+            }
+        }
+        Ok(())
+    }
+
     /// Notify the engine that a document has been inserted.
     /// Checks unique constraints and updates all index structures.
     pub fn on_insert(&mut self, doc: &Document) -> Result<(), IndexError> {