add TERMINFO env for top/ncurses, tighten output flush

top fails with "Error opening terminal: xterm-256color" because the
PTY child can't find terminfo. Set TERMINFO=/usr/share/terminfo
explicitly in the pane environment.

Output flush changed from 16ms Timer to Duration.zero (next event
loop turn) for tighter batching of escape sequences.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/src/panes/registry.dart

@@ -56,6 +56,7 @@ class PaneRegistry {
       'COLORTERM': 'truecolor',
       'LANG': 'en_US.UTF-8',
       'LC_ALL': 'en_US.UTF-8',
+      'TERMINFO': '/usr/share/terminfo',
       if (env != null) ...env,
     };