Replaces the single-crate Cargo.toml with a workspace containing
ahfail-gtklock (migrated from root src/) and three stub crates
(ahfail-ui, ahfail-pam, ahfail-display). Updates meson.build to
build with -p ahfail-gtklock.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
