test_serial mocked log_write.h, which serial.c never calls.
Split celrs_serial into celrs_serial (platform-agnostic logic)
and celrs_serial_platform (real Win/POSIX backend), matching
the celrs_logger/celrs_log_write split.
test_serial now mocks celrs/platform/serial_internal.h and
links only celrs_serial, so the list-ports tests verify the
max_ports clamping and pass-through logic without hitting the
real registry or /dev.
portersky