Rewrite moonlock from Python to Rust (v0.4.0)

Complete rewrite of the Wayland lockscreen from Python/PyGObject to
Rust/gtk4-rs for memory safety in security-critical PAM code and
consistency with the moonset/moongreet Rust ecosystem.

Modules: main, lockscreen, auth (PAM FFI), fingerprint (fprintd D-Bus),
config, i18n, users, power. 37 unit tests.

Security: PAM conversation callback with Zeroizing password, panic hook
that never unlocks, root check, ext-session-lock-v1 compositor policy,
absolute loginctl path, avatar symlink rejection.

.gitignore

@@ -1,10 +1,4 @@
-__pycache__/
-*.pyc
-.venv/
-*.egg-info/
-dist/
-build/
-.pytest_cache/
+/target
 
 # makepkg build artifacts
 pkg/src/