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.

build.rs

@@ -0,0 +1,13 @@
+// ABOUTME: Build script for compiling GResource bundle.
+// ABOUTME: Bundles style.css, wallpaper.jpg, and default-avatar.svg into the binary.
+
+fn main() {
+    glib_build_tools::compile_resources(
+        &["resources"],
+        "resources/resources.gresource.xml",
+        "moonlock.gresource",
+    );
+
+    // Link libpam for PAM authentication
+    println!("cargo:rustc-link-lib=pam");
+}