Add cel_serial_platform_open/close/read/write/flush for
Windows using CreateFileA, DCB for baud/8N1, and
SetCommTimeouts for non-blocking reads.
serial.c now delegates all operations to the platform
backend via a cel_serial_platform_handle, and
test_serial.c mocks that backend with CMock.
Drop timeout_ms; read now returns immediately with whatever
data is available (0 if none), so callers don't block the rest
of their loop waiting on serial I/O.
telemetry's poll loop now sleeps interval_ms itself between
empty reads via a small sleep_ms helper.
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.
Implement cel_serial_list_ports/cel_serial_free_ports with
platform backends: Windows reads HKLM\HARDWARE\DEVICEMAP\
SERIALCOMM (fast, single registry read), POSIX scans /dev for
ttyUSB*/ttyACM*.
telemetry tool gains --list, --port, and --baudrate flags;
baud rate was previously hardcoded to 400000. Rename the
tool_telemetry CMake target to telemetry.
Fix test_free_ports_zero_count, which passed a stack array to
cel_serial_free_ports (which calls free() on it), corrupting
the heap.
portersky