Add client registry and CLIENT_INFO UART command on master.

Track ESP-NOW slaves in a shared registry and respond to CLIENT_INFO
with protobuf ClientInfoResponse; ESP-NOW path upserts registry entries.

Co-authored-by: Cursor <cursoragent@cursor.com>

main/client_registry.h

@@ -0,0 +1,33 @@
+#ifndef CLIENT_REGISTRY_H
+#define CLIENT_REGISTRY_H
+
+#include "esp_err.h"
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+#define CLIENT_REGISTRY_MAX 16
+#define CLIENT_MAC_LEN 6
+
+typedef struct {
+  uint32_t id;
+  bool available;
+  bool used;
+  uint8_t mac[CLIENT_MAC_LEN];
+  uint32_t last_ping;
+  uint32_t last_success_ping;
+  uint32_t version;
+} client_info_t;
+
+void client_registry_init(void);
+
+/** Register or refresh a client; sets last_ping and last_success_ping to now. */
+esp_err_t client_registry_upsert(const uint8_t mac[CLIENT_MAC_LEN], uint32_t id,
+                                 uint32_t version, bool available, bool used,
+                                 bool *out_is_new);
+
+size_t client_registry_count(void);
+const client_info_t *client_registry_at(size_t index);
+const client_info_t *client_registry_find_by_mac(const uint8_t mac[CLIENT_MAC_LEN]);
+
+#endif