Use threadsafe death tests in Release builds
ThreadPool.MultiThreadException took 3,552 seconds in the Protein static CI job, where GoogleTest warned about forking a process with 10–11 threads.
Use the existing Debug threadsafe death-test mode in Release builds too, so death tests re-execute with fresh process state. This follows GoogleTest's guidance for death tests and threads. No tests or assertions are removed.
Validation on mex: the updated test entry point was compiled and linked into #670's GCC 13 Release build; all 3,709 unit cases passed with no fork warnings. This includes the 13 cursor tests beyond master. Fresh CI passed: 49 jobs succeeded, with two intentional skips.
The new Protein static CI job passed all 3,001 tests. The death test took 2.6 seconds with no fork warnings; the entire job's unit run took 561 seconds. This is an observation from separate runners, not a controlled timing comparison. The full workflow, including the container build, passed.