feat(cli): probe the image tooling

The run path shells out to mkfs.ext4 and debugfs from e2fsprogs; a
host without them fails at the first pull, so probe names them up
front like it names KVM and the engine.

crates/ecrou-cli/src/main.rs

@@ -142,6 +142,23 @@ fn cmd_probe() -> std::process::ExitCode {
         }
     }
 
+    print!("e2fsprogs:    ");
+    let mut tools_ok = true;
+    for tool in ["mkfs.ext4", "debugfs"] {
+        match std::process::Command::new(tool).arg("-V").output() {
+            Ok(_) => {}
+            Err(err) => {
+                ok = false;
+                tools_ok = false;
+                println!("missing {tool} ({err}); the rootfs image cannot be built");
+                break;
+            }
+        }
+    }
+    if tools_ok {
+        println!("ok");
+    }
+
     print!("cache:        ");
     match ecrou::cache::RootfsCache::default_dir() {
         Ok(dir) => {