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.

src/power.rs

@@ -0,0 +1,52 @@
+// ABOUTME: Power actions — reboot and shutdown via loginctl.
+// ABOUTME: Wrappers around system commands for the lockscreen UI.
+
+use std::fmt;
+use std::process::Command;
+
+#[derive(Debug)]
+pub enum PowerError {
+    CommandFailed { action: &'static str, message: String },
+    Timeout { action: &'static str },
+}
+
+impl fmt::Display for PowerError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            PowerError::CommandFailed { action, message } => write!(f, "{action} failed: {message}"),
+            PowerError::Timeout { action } => write!(f, "{action} timed out"),
+        }
+    }
+}
+
+impl std::error::Error for PowerError {}
+
+fn run_command(action: &'static str, program: &str, args: &[&str]) -> Result<(), PowerError> {
+    let child = Command::new(program)
+        .args(args)
+        .spawn()
+        .map_err(|e| PowerError::CommandFailed { action, message: e.to_string() })?;
+    let output = child.wait_with_output()
+        .map_err(|e| PowerError::CommandFailed { action, message: e.to_string() })?;
+    if !output.status.success() {
+        let stderr = String::from_utf8_lossy(&output.stderr);
+        return Err(PowerError::CommandFailed {
+            action, message: format!("exit code {}: {}", output.status, stderr.trim()),
+        });
+    }
+    Ok(())
+}
+
+pub fn reboot() -> Result<(), PowerError> { run_command("reboot", "/usr/bin/loginctl", &["reboot"]) }
+pub fn shutdown() -> Result<(), PowerError> { run_command("shutdown", "/usr/bin/loginctl", &["poweroff"]) }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test] fn power_error_display() { assert_eq!(PowerError::CommandFailed { action: "reboot", message: "fail".into() }.to_string(), "reboot failed: fail"); }
+    #[test] fn timeout_display() { assert_eq!(PowerError::Timeout { action: "shutdown" }.to_string(), "shutdown timed out"); }
+    #[test] fn missing_binary() { assert!(run_command("test", "nonexistent-xyz", &[]).is_err()); }
+    #[test] fn nonzero_exit() { assert!(run_command("test", "false", &[]).is_err()); }
+    #[test] fn success() { assert!(run_command("test", "true", &[]).is_ok()); }
+}