- u_async.c: process immediate_queue BEFORE epoll_wait in uasync_poll
- stcp_link.c: defensive NULL etcp_conn/etcp_link in stcp_link_close
- utun_instance.c: move stcp_server_list_destroy_all to Phase J (before ETCP),
add deferred drain in both Phase J and L
- stcp.h/c: STCP_HS_ENC_CLIENT 6→38, STCP_HS_ENC_SERVER 7→39,
exchange ed25519_pubkey in both handshake directions
- etcp_connections.c: copy peer_ed25519 from STCP to ETCP_CONN in
etcp_link_enter_ready_tcp; guard link->etcp in tcp_link_close_cb
- chat_sync.c: fix lk->conn NULL dereference for TCP links
- test_stcp.c: update stcp_server_create/stcp_client_connect callsites
- tools/chat_tcp_test: change transport udp→tcp
- uasync_set_timeout(0): use FIFO immediate_queue instead of timeout_heap
to guarantee execution AFTER all currently-expired heap timers
- uasync_cancel_timeout: search immediate_queue for timeout=0 timers
- dummynet: start initial shaper with bandwidth-based delay instead of 0
(prevents queue from never filling up when packets arrive sequentially)
- test_dummynet: rename get_time_us -> now_us (conflict with u_async.h)
- test_u_async_comprehensive: fix immediate_timeouts, add timeout_ordering test