Rewrite moongreet from Python to Rust (v0.3.0)

Complete rewrite of the greetd greeter from Python/PyGObject to Rust/gtk4-rs
for consistency with moonset, single binary without Python runtime, and
improved security through Rust memory safety.

Modules: main, greeter, ipc, config, i18n, users, sessions, power
86 unit tests covering all modules including login_worker IPC flow.
Security hardening: shell-word splitting for exec_cmd, absolute path
validation for session binaries, session-name sanitization, absolute
loginctl path, atomic IPC writes.

Cargo.toml

@@ -0,0 +1,25 @@
+[package]
+name = "moongreet"
+version = "0.3.0"
+edition = "2024"
+description = "A greetd greeter for Wayland with GTK4 and Layer Shell"
+license = "MIT"
+
+[dependencies]
+gtk4 = { version = "0.11", features = ["v4_10"] }
+gtk4-layer-shell = "0.8"
+glib = "0.22"
+gdk4 = "0.11"
+gdk-pixbuf = "0.22"
+gio = "0.22"
+toml = "0.8"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+log = "0.4"
+env_logger = "0.11"
+
+[dev-dependencies]
+tempfile = "3"
+
+[build-dependencies]
+glib-build-tools = "0.22"