New shared crate qbz-slint-common (will also host the artwork primitives via a
dependency-inversion pass). toast.rs is a clean leaf over qbz-ui: only Slint
types (AppWindow, ToastKind, ToastState) + slint core (Timer/Weak). Moved
verbatim; the only edit is the import (`crate::` -> `qbz_ui::`). Bin re-exports
via `pub use qbz_slint_common::toast;` so all 154 call sites across 21 files are
unchanged. slint dep is `default-features = false` (adversarial review: a
default-features slint here would turn on the default renderer for the whole
build via feature unification). `cargo check -p qbz` green.
cc
vicrodh
d1437c70eb