headers: add extern "C" guards for C++ interop (chatgui)

Added #ifdef __cplusplus / extern "C" { ... } / #endif to ~65 headers
in src/ and lib/, enabling the C library to be linked into C++ code (chatgui).
Fixed packet_dump.h (added missing header guard) and etcp_bbr.h
(#pragma once guard).

lib/ll_queue.h

@@ -1,6 +1,11 @@
 #ifndef LL_QUEUE_H
 #define LL_QUEUE_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
 #include <stddef.h> // для size_t
 #include <stdint.h>  // для uint64_t
 #include "memory_pool.h" // для struct memory_pool
@@ -412,4 +417,8 @@ static inline size_t queue_total_bytes(struct ll_queue* q) {
     return q->total_bytes;
 }
 
+
+#ifdef __cplusplus
+}
+#endif
 #endif // LL_QUEUE_H