stcp: ed25519 pubkey exchange in handshake, fix instance destroy segfault, fix chat_tcp_test

- 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

tests/test_offset.c

@@ -0,0 +1,41 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include "../lib/mem.h"
+
+typedef struct {
+    int a;
+    int b;
+    int ref_count;
+    uint8_t data[0];
+} test_struct;
+
+int main() {
+    test_struct* entry = u_malloc(sizeof(test_struct) + 100);
+    entry->a = 1;
+    entry->b = 2;
+    entry->ref_count = 3;
+    
+    void* data_ptr = entry->data;  // or (void*)(entry + 1)
+    
+    printf("entry pointer: %p\n", entry);
+    printf("data pointer: %p\n", data_ptr);
+    printf("sizeof(test_struct): %zu\n", sizeof(test_struct));
+    printf("difference in bytes: %ld\n", (char*)data_ptr - (char*)entry);
+    
+    // Test different ways to convert back
+    test_struct* back1 = (test_struct*)((char*)data_ptr - sizeof(test_struct));
+    test_struct* back2 = (test_struct*)data_ptr - 1;
+    test_struct* back3 = (test_struct*)data_ptr - 0;
+    
+    printf("back1 (char* subtraction): %p\n", back1);
+    printf("back2 (pointer - 1): %p\n", back2);
+    printf("back3 (pointer - 0): %p\n", back3);
+    
+    printf("back1 ref_count: %d\n", back1->ref_count);
+    printf("back2 ref_count: %d\n", back2->ref_count);
+    printf("back3 ref_count: %d\n", back3->ref_count);
+    
+    u_free(entry);
+    return 0;
+}