* fix(ibus): start engine process before checking registration
_setup_engine() gated on is_engine_registered() which queries
`ibus list-engine`. On systems where the IBus daemon doesn't scan
user-level component directories (~/.local/share/ibus/component/),
the engine never appeared in the list, blocking start_engine_process()
from ever being called — even though its register_component() D-Bus
call is the correct registration path for standalone-launched engines.
Reorder _setup_engine() to start the engine process first, relying on
register_component() for runtime registration. Remove the XML
install step from the setup path since install_ibus_component()
triggers `ibus restart` which kills the engine process.
Follow-up to PR #304 which added register_component() support.
* fix(ibus): remove dead code and add engine readiness check
Address review feedback:
- Remove install_ibus_component(), is_engine_registered(),
is_component_up_to_date(), _get_expected_component_xml()
and their tests — no longer called after setup order fix
- Add SOCKET_PATH poll after start_engine_process() to verify
engine is fully initialized before activation
- Rebase onto current main
