New category (id=25) replaces DEBUG_CATEGORY_DEBUG in chat_sync and
chat_core. This allows enabling only protocol-level logs (SEND/RECV,
timeouts, handshake) without the noise from DEBUG category.
Changes:
- debug_config.h: add CONNECTIVITY=25, COUNT→26
- debug_config.c: add 'connectivity' to g_debug_categories
- chat_sync.c: DEBUG→CONNECTIVITY in all protocol logs
- chat_core.c: DEBUG→CONNECTIVITY in connection/create logs
- utun_node.cpp: remove hardcoded DEBUG=TRACE (now config-driven)
- chatgui.cfg: connectivity=info in debug_categories