fix: --kernel =PATH syntax, exit-code cleanup, bind warning in VM mode

- --kernel took an optional value greedily, swallowing the DISTRO
  positional: 'ecr --kernel alpine' failed with a missing-argument error
  and 'ecr --kernel alpine -- cmd' pulled a garbage OCI ref. require
  =PATH syntax for the kernel path instead.
- setup_namespaces returns the child's exit code (or 128+signal) instead
  of calling process::exit, so the extracted rootfs tempdir is cleaned
  up before exiting instead of leaking into /tmp.
- warn that --bind/--bind-rw are ignored when booting with --kernel.

src/namespace.rs

@@ -58,8 +58,10 @@ pub fn check_user_namespace() -> Result<()> {
     Ok(())
 }
 
-/// Setup namespaces and run the provided function inside them
-pub fn setup_namespaces<F>(f: F) -> Result<()>
+/// Setup namespaces and run the provided function inside them.
+/// Returns the child's exit code (0 on success, 128+signal when killed by a
+/// signal). Setup failures are returned as Err.
+pub fn setup_namespaces<F>(f: F) -> Result<i32>
 where
     F: FnOnce() -> Result<()> + Send + 'static,
 {
@@ -222,20 +224,18 @@ where
     let status = nix::sys::wait::waitpid(pid, None)?;
 
     match status {
-        nix::sys::wait::WaitStatus::Exited(_, 0) => Ok(()),
+        nix::sys::wait::WaitStatus::Exited(_, 0) => Ok(0),
         nix::sys::wait::WaitStatus::Exited(_, code) => {
             // If the child reported an error (e.g., setup failure), return it.
             // Otherwise, just forward the exit code without an error message.
             if let Some(msg) = child_error {
                 Err(anyhow!("{}", msg))
             } else {
-                std::process::exit(code);
+                Ok(code)
             }
         }
-        nix::sys::wait::WaitStatus::Signaled(_, sig, _) => {
-            Err(anyhow!("Child process killed by signal {:?}", sig))
-        }
-        _ => Ok(()),
+        nix::sys::wait::WaitStatus::Signaled(_, sig, _) => Ok(128 + sig as i32),
+        _ => Ok(0),
     }
 }