workspace: add bread-app crate and first-cut bread-polkit agent

bread-app is the GTK bootstrap new tools should use instead of another
copied main.rs: com.breadway.* app id, singleton lock, optional
gtk_popup re-export, optional bread.command.<app>.** listen loop.
Tests cover app-id helpers and command-verb parse. Existing apps are
not migrated.

bread-polkit is an own PolicyKit1 session authentication agent with a
bread-theme GTK4 password prompt (not a polkit-gnome wrapper).
Autostart via contrib/bread-polkit.desktop or exec-once. Not a bakery
product; not added to the BOS ISO lockfile.

bread-polkit/src/lib.rs

@@ -0,0 +1,10 @@
+//! Non-GTK PolicyKit helper logic for `bread-polkit`.
+//!
+//! The binary (`bread-polkit`) registers as a session authentication
+//! agent and shows a themed password prompt. This library is the
+//! transport / identity / session parsing that can be unit-tested
+//! without a display.
+
+pub mod helper;
+pub mod identity;
+pub mod session;