Separate floating pane border label from pane title

--border-label of a tmux floating pane was stored in the pane title and
read back by pane-border-format as '#{pane_title}', so any program in the
pane could replace the border text by setting the title. On Zellij the
name was only set when the pane was created. Either way the built-in
label actions could not touch it, as fzf draws no border of its own
there.

- tmux: hold the label in pane-scoped @fzf-border-label. Set the option
  even without a label, so a later change-border-label has somewhere to
  write. Drop select-pane -T; the pane title is left to the user
- Zellij: run 'zellij action rename-pane' on label change. 'zellij run'
  has no option to set the environment of the new pane, so the command
  exports the target itself
- change-border-label and transform-border-label now update the native
  border, through the setter nativeLabelSetter picks by multiplexer
- The pane is named in a __FZF_INTERNAL_ variable that runProxy withholds
  from the environment replay, so the same name in the outer environment
  cannot redirect the update to another pane

man/man1/fzf.1

@@ -426,9 +426,13 @@ On tmux 3.7 or above and on Zellij, the floating pane is not modal; you can
 switch to other panes and windows while fzf is running, and move and resize
 the pane with the mouse. The native border of the pane is the handle for
 moving and resizing it, so it is used by default and \fBborder\-native\fR is
-implied. On tmux, \fB\-\-border\-label\fR is set as the title of the pane,
-and is displayed on the border if \fBpane\-border\-status\fR is enabled in
-tmux (\fB\-\-border\-label\-pos\fR is ignored).
+implied. \fB\-\-border\-label\fR is displayed on the native border, and
+\fBchange\-border\-label\fR and \fBtransform\-border\-label\fR update it
+(\fB\-\-border\-label\-pos\fR is ignored). On tmux, fzf holds the label in
+the \fB@fzf\-border\-label\fR option of the pane and sets its
+\fBpane\-border\-format\fR to read it back, so the label is displayed if
+\fBpane\-border\-status\fR is enabled in tmux. The title of the pane is left
+alone. On Zellij, the label is the name of the pane.
 
 fzf draws its own border instead when a border style is explicitly specified
 with \fB\-\-border\fR, so that it is the only border shown. \fBnone\fR and