First increment of M1 of the Slint functional POC. Adds the qbz-slint
workspace member: Slint 1.x dependency, build.rs compiling ui/app.slint,
a minimal AppWindow, and the binary entry point.
Verifies the Slint toolchain builds in this environment before the
foundation layer (tokens, shell, theme, i18n, typed commands) lands.
Private slint-poc branch only (ADR-007).
cc
