Discard RF monitor frames for background sessions on screen flip

Background sessions no longer buffer monitor frames in mon_sbuf.
BBS output (out_sbuf) is still buffered so you see what the node
sent while you were away. The monitor starts live/fresh when you
switch sessions instead of replaying a backlog of RF traffic.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

termtcp.c

@@ -954,10 +954,11 @@ static void scrl_draw_window(WINDOW *win, ScrlLine *buf, int count, int head,
 void append_mon(const char *text, int len, int attr)
 {
     if (!mon_visible || len <= 0) return;
+    if (suppress_draw) return;   /* background session — discard RF monitor backlog */
 
     const char *p   = text;
     const char *end = text + len;
-    int live = (mon_win != NULL && !suppress_draw);
+    int live = (mon_win != NULL);
 
     S->mon_part_cur = attr;
     if (S->mon_part_len == 0) S->mon_part_base = attr;