LayoutStore.Load threw ArgumentOutOfRangeException on GetOrdinal("BlurAmount") during the
MainViewModel constructor, so the app died at startup rather than when a blur was used. The
migration adds the column and Save writes it; the reading query still listed its columns
explicitly and BlurAmount was missing from that list.
Adds LayoutBlurPersistenceTests: a blur saved and reloaded, asserting the amount and geometry
survive. That is the test that should have existed with the feature — the startup path it
covers has no other guard.
