build: extract libutun.a from src/ + add db_sync module + etcp API, BBR improvements

- Add libutun.a (noinst) containing all src/*.c except utun.c
- utun binary now links libutun.a + libuasync.a
- tests/Makefile.am simplified: all tests link libutun.a instead of
  manual .o file lists (removed ~70 lines of boilerplate)
- Add db_sync module: distributed content-addressed DB with LMDB
  backend, peer-to-peer sync via etcp_router (svc 0x20)
- etcp API: etcp_send_in_order(), etcp_clear_inflight_queues(),
  verify_packet_signature(), BBR getters
- BBR: adaptive threshold (1/4 of pipe-min instead of hardcoded)
- route_bgp/etcp_connect: STCP transport support in NODEINFO
- conn_mgr: NODEINFO_ADDRS doc for nat_type/transport/proto fields
- Cleanup: removed AUDIT_FINDINGS.txt, filelist.txt, nat.txt

src/config_parser.c

@@ -399,6 +399,14 @@ static int parse_global(const char *key, const char *value, struct global_config
     if (strcmp(key, "db_path") == 0) {
         return assign_string(global->db_path, sizeof(global->db_path), value);
     }
+    if (strcmp(key, "db_sync_enabled") == 0) {
+        global->db_sync_enabled = atoi(value);
+        return 0;
+    }
+    if (strcmp(key, "db_sync_ttl") == 0) {
+        global->db_sync_ttl = (uint32_t)atoi(value);
+        return 0;
+    }
     /* debug_categories key is deprecated - use [debug] section instead */
     if (strcmp(key, "enable_timestamp") == 0) {
         global->enable_timestamp = atoi(value);
@@ -792,6 +800,8 @@ static struct utun_config* parse_config_internal(FILE *fp, const char *filename)
     cfg->global.firewall_bypass_all = 0;
     cfg->global.control_allows = NULL;
     cfg->global.control_allow_count = 0;
+    cfg->global.db_sync_enabled = 0;
+    cfg->global.db_sync_ttl = 86400;
 
     section_type_t cur_section = SECTION_UNKNOWN;
     struct CFG_SERVER *cur_server = NULL;