v0.7.9

Added

• Record::create() is the one way a record comes into being. Under a lock it runs a guard that refuses a conflict, writes a minimal index.json holding the uuid, builds the record and applies the data through save().
• A race script in tests/stress/ that creates the same record from several processes.

Changed

• Page::new(), User::new() and the collection field's item creation go through Record::create(). A duplicate slug, username or item uuid throws ConflictException instead of racing.