TMUX is not forwarded over SSH, so a TUI launched on a remote host from
inside local tmux only sees TERM=tmux/tmux-256color with no TMUX var --
the cursor-drift bug still applies there. Extend supportsFastEchoTerminal()
to also fall back when TERM is tmux-flavored.
Deliberately scoped to tmux* only, NOT screen*: GNU screen sets the same
screen/screen-256color TERM and has no reported drift, so widening to
screen would disable the optimization for those users with no evidence of
a bug (matching the original PR's stated out-of-scope note).
Adds tests for tmux-flavored TERM (disabled) and screen/xterm TERM
(stays enabled) to guard against accidental widening.
e52fffb607