init: replace .expect("TODO") on namespace registration with descriptive panic messages

The scheme-service boot path in service.rs used .expect("TODO") on two
boot-critical libredox calls:

  - libredox::call::getns() — gets a fd to the current namespace
  - libredox::call::register_scheme_to_ns() — registers the spawned
    scheme into that namespace

If either call fails the scheme is invisible, which is non-recoverable
for boot.  The placeholder "TODO" message gave no diagnostic value.

Replaced with descriptive messages:
  - getns:         .expect("init: failed to get current namespace fd; ...")
  - register_scheme: unwrap_or_else(|e| panic!(...)) including the
    scheme name and the error, matching the existing pattern at line 87
    (create_pipe unwrap_or_else).

No behaviour change — both paths still panic on failure.  The only
difference is that the panic message now identifies the failure and
includes the offending scheme name.

init/src/service.rs

@@ -137,9 +137,15 @@ impl Service {
                     }
                 }
 
-                let current_namespace_fd = libredox::call::getns().expect("TODO");
+                let current_namespace_fd = libredox::call::getns()
+                    .expect("init: failed to get current namespace fd; boot cannot continue");
                 libredox::call::register_scheme_to_ns(current_namespace_fd, scheme, new_fd)
-                    .expect("TODO");
+                    .unwrap_or_else(|e| {
+                        panic!(
+                            "init: failed to register scheme '{}' in namespace: {}; boot cannot continue",
+                            scheme, e
+                        )
+                    });
             }
             ServiceType::Oneshot => {
                 drop(read_pipe);