feat: add serial port listing and CLI flags

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.

tests/test_serial.c

@@ -1,3 +1,5 @@
+#include <stdlib.h>
+
 #include "unity.h"
 #include "celrs/serial.h"
 #include "Mocklog_write.h"
@@ -58,6 +60,34 @@ void test_flush_null(void) {
     cel_serial_flush(NULL); /* should not crash */
 }
 
+void test_list_ports_null_out(void) {
+    TEST_ASSERT_EQUAL_INT(-1, cel_serial_list_ports(NULL, 16));
+}
+
+void test_list_ports_runs_without_crash(void) {
+    char** ports = NULL;
+    int count = cel_serial_list_ports(&ports, 16);
+    /* count >= 0 is valid (may be 0 if no ports available) */
+    TEST_ASSERT_GREATER_OR_EQUAL_INT(0, count);
+    cel_serial_free_ports(ports, count);
+}
+
+void test_list_ports_zero_max_uses_default(void) {
+    char** ports = NULL;
+    int count = cel_serial_list_ports(&ports, 0);
+    TEST_ASSERT_GREATER_OR_EQUAL_INT(0, count);
+    cel_serial_free_ports(ports, count);
+}
+
+void test_free_ports_null(void) {
+    cel_serial_free_ports(NULL, 0); /* should not crash */
+}
+
+void test_free_ports_zero_count(void) {
+    char** empty = (char**)calloc(1, sizeof(char*));
+    cel_serial_free_ports(empty, 0); /* should not crash */
+}
+
 int main(void) {
     UNITY_BEGIN();
     RUN_TEST(test_open_valid_path);
@@ -68,5 +98,10 @@ int main(void) {
     RUN_TEST(test_write_returns_zero_stub);
     RUN_TEST(test_flush_no_crash);
     RUN_TEST(test_flush_null);
+    RUN_TEST(test_list_ports_null_out);
+    RUN_TEST(test_list_ports_runs_without_crash);
+    RUN_TEST(test_list_ports_zero_max_uses_default);
+    RUN_TEST(test_free_ports_null);
+    RUN_TEST(test_free_ports_zero_count);
     return UNITY_END();
 }