Removing a tag from an item doesn't stick

Found by the VAULT-42 differential test, which compares the new record store with the SwiftData store.

The bug: when an item loses a tag, the tag stays on the item. It happens in two places:

• editing an item and removing one of its tags
• a merge import that brings in a newer copy of an item with fewer tags

It reproduces on the SQLite store the app uses, including after reopening it.

Cause: PersistedLocalVaultStore.update inserted a new model with the same id and relied on SwiftData's unique-id upsert. The upsert merges to-many relationships (tags) instead of replacing them.

Fix: the store updates items and tags in place. It replaces the tag set, updates a detail of the same kind in place, and deletes a detail the item no longer has, so no orphaned detail rows (old secrets) are left behind. The contract suite covers removing tags on update, changing an item's kind, and a newer import replacing tags.