test: stop native IPC fixture without restart

[breznknecht]

Summary

• stop the ExUnit-managed native IPC supervisor through stop_supervised/1 so its permanent child specification is removed
• prevent endpoint-death tests from racing an automatic replacement core publication
• assert that the endpoint descriptor is gone before classifying the admitted operation disconnect
• preserve the strict distinction between indeterminate disconnects and confirmed core_replaced results

Root cause

The tests called Supervisor.stop/1 on a child started with start_supervised!/1. ExUnit retained the permanent child specification and restarted the endpoint with a new core_instance_id. The Swift helper then correctly returned either indeterminate or core_replaced depending on whether it read the descriptor before or after the replacement published.

Validation

• native IPC helper integration module: 12 tests passed
• formerly flaky disconnect test: 100 consecutive passes with the exact indeterminate assertion unchanged
• make lint
• mix format --check-formatted test/macos/native_ipc_helper_integration_test.exs
• git diff --check
• final acceptance review: PASS, no warnings