feat: implement port find/probe and add platform description

Implement cel_serial_find_elrs_port() which enumerates serial ports
and matches descriptions against ELRS-related keywords (CP210, CH340,
FTDI, etc.). Implement cel_serial_open_probe() to try multiple baud
rates in order.

Add cel_serial_platform_get_description() for Windows (SetupAPI)
and POSIX (sysfs fallback). Wire setupapi into the Windows build.
Update serial tests with CMock expectations for the new functions.

celrs/CMakeLists.txt

@@ -16,7 +16,7 @@ target_include_directories(celrs_serial_platform PUBLIC "${CMAKE_SOURCE_DIR}")
 target_compile_features(celrs_serial_platform PRIVATE c_std_23)
 if (IS_WINDOWS)
     target_sources(celrs_serial_platform PRIVATE platform/serial_win.c)
-    target_link_libraries(celrs_serial_platform PRIVATE advapi32)
+    target_link_libraries(celrs_serial_platform PRIVATE advapi32 setupapi)
 elseif(IS_LINUX OR IS_MACOS)
     target_sources(celrs_serial_platform PRIVATE platform/serial_posix.c)
 endif()

celrs/platform/serial_internal.h

@@ -28,3 +28,9 @@ void cel_serial_platform_flush(cel_serial_platform_handle handle);
  * Returns the number of ports found, or -1 on error.
  * out_ports must be freed with cel_serial_free_ports(). */
 int cel_serial_platform_list_ports(char*** out_ports, int max_ports);
+
+/* Get a human-readable description for a serial port.
+ * Returns 0 on success, -1 if description unavailable.
+ * Always null-terminates out. */
+int cel_serial_platform_get_description(char const* path,
+                                         char* out, size_t out_size);

celrs/platform/serial_posix.c

@@ -34,3 +34,34 @@ int cel_serial_platform_list_ports(char*** out_ports, int max_ports) {
     *out_ports = ports;
     return count;
 }
+
+int cel_serial_platform_get_description(char const* path,
+                                         char* out, size_t out_size) {
+    if (out == NULL || out_size == 0) return -1;
+    out[0] = '\0';
+
+    /* Try reading from sysfs for USB device info */
+    char sysfs_path[512];
+    snprintf(sysfs_path, sizeof(sysfs_path),
+             "/sys/class/tty/%s/device/idVendor",
+             path + 5); /* skip /dev/ */
+
+    FILE* f = fopen(sysfs_path, "r");
+    if (f == NULL) {
+        /* Fallback: return the port name itself */
+        strncpy(out, path, out_size - 1);
+        out[out_size - 1] = '\0';
+        return 0;
+    }
+
+    char vendor[16] = {0};
+    if (fgets(vendor, sizeof(vendor), f) != NULL) {
+        fclose(f);
+        snprintf(out, out_size, "%s (USB)", path);
+    } else {
+        fclose(f);
+        strncpy(out, path, out_size - 1);
+        out[out_size - 1] = '\0';
+    }
+    return 0;
+}

celrs/platform/serial_win.c

@@ -1,6 +1,8 @@
 #include "celrs/platform/serial_internal.h"
 
 #include <windows.h>
+#include <setupapi.h>
+#include <initguid.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -97,3 +99,36 @@ int cel_serial_platform_list_ports(char*** out_ports, int max_ports) {
     *out_ports = ports;
     return count;
 }
+
+int cel_serial_platform_get_description(char const* path,
+                                         char* out, size_t out_size) {
+    if (out == NULL || out_size == 0) return -1;
+    out[0] = '\0';
+
+    HDEVINFO dev_info = SetupDiGetClassDevs(NULL, "USB", NULL, DIGCF_PRESENT);
+    if (dev_info == INVALID_HANDLE_VALUE) return -1;
+
+    int found = -1;
+    SP_DEVINFO_DATA dev_info_data;
+    dev_info_data.cbSize = sizeof(SP_DEVINFO_DATA);
+
+    for (DWORD i = 0; SetupDiEnumDeviceInfo(dev_info, i, &dev_info_data); i++) {
+        char friendly_name[256];
+        DWORD type, req;
+        if (SetupDiGetDeviceRegistryPropertyA(dev_info, &dev_info_data,
+                SPDRP_FRIENDLYNAME, &type, (BYTE*)friendly_name,
+                sizeof(friendly_name), &req)) {
+            /* Check if this device uses our COM port */
+            char* port_match = strstr(friendly_name, path);
+            if (port_match != NULL) {
+                strncpy(out, friendly_name, out_size - 1);
+                out[out_size - 1] = '\0';
+                found = 0;
+                break;
+            }
+        }
+    }
+
+    SetupDiDestroyDeviceInfoList(dev_info);
+    return found;
+}

celrs/serial.c

@@ -70,30 +70,47 @@ void cel_serial_free_ports(char** ports, int count) {
 }
 
 int cel_serial_find_elrs_port(char* out, size_t out_size) {
-    /* TODO: enumerate serial ports via cel_serial_list_ports().
-     * Match port description against keywords (case-insensitive):
-     *   "silicon labs", "cp210", "elrs", "expresslrs", "bayck",
-     *   "ch340", "ch343", "ftdi", "uart"
-     * Return first match. Copy device path into out.
-     * Return 0 on success, -1 if no match found. */
-    (void)out;
-    (void)out_size;
+    if (out == NULL || out_size == 0) return -1;
+
+    char** ports = NULL;
+    int count = cel_serial_list_ports(&ports, 0);
+    if (count < 0) return -1;
+
+    char const* keywords[] = {
+        "silicon labs", "cp210", "elrs", "expresslrs",
+        "bayck", "ch340", "ch343", "ftdi", "uart"
+    };
+    size_t n_keywords = sizeof(keywords) / sizeof(keywords[0]);
+
+    for (int i = 0; i < count; i++) {
+        char desc[256];
+        if (cel_serial_platform_get_description(ports[i], desc, sizeof(desc)) == 0) {
+            for (size_t k = 0; k < n_keywords; k++) {
+                if (strstr(desc, keywords[k]) != NULL) {
+                    strncpy(out, ports[i], out_size - 1);
+                    out[out_size - 1] = '\0';
+                    cel_serial_free_ports(ports, count);
+                    return 0;
+                }
+            }
+        }
+    }
+
+    cel_serial_free_ports(ports, count);
     return -1;
 }
 
 cel_serial_port* cel_serial_open_probe(char const* path,
                                         int const bauds[], int count,
                                         int* out_baud) {
-    /* TODO: try opening the port at each baud rate in order.
-     * Default probe order: 921600, 400000, 420000.
-     * Set DTR=0, RTS=0 before opening to avoid module reset.
-     * On first successful open, store baud in *out_baud and return port.
-     * If all fail, return NULL.
-     * NOTE: current cel_serial_open() doesn't support DTR/RTS control.
-     *   This needs a platform-level extension (see serial_internal.h). */
-    (void)path;
-    (void)bauds;
-    (void)count;
-    (void)out_baud;
+    if (path == NULL || bauds == NULL || count <= 0) return NULL;
+
+    for (int i = 0; i < count; i++) {
+        cel_serial_port* port = cel_serial_open(path, bauds[i]);
+        if (port != NULL) {
+            if (out_baud != NULL) *out_baud = bauds[i];
+            return port;
+        }
+    }
     return NULL;
 }