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.h

@@ -115,6 +115,8 @@ struct global_config {
     // Debug and logging configuration
     char log_file[256];        // Path to log file (empty = stdout)
     char db_path[256];         // Path to LMDB nodeinfo database (empty = disabled)
+    int  db_sync_enabled;      // 1 = enable distributed DB sync (default: 0)
+    uint32_t db_sync_ttl;      // TTL for unsent records in seconds (default: 86400)
     char debug_level[16];      // debug level: error, warn, info, debug, trace
     int enable_timestamp;      // enable timestamps in logs
     int enable_function_names; // enable function names in logs