Apply facing-line rule to input border as well

Visible input border suppressed separator even when it draws no line
toward list section (bottom in default layout, top in reverse, vertical).
Suppress only when input border has a line on the list side; otherwise
fall through to adjacent section check.

- Requires header window to have content before treating it as facing
  section; hasHeaderWindow returns true with empty header when input
  border is visible
- Extract inputBorderFacesList and resolveInputBorderShape shared by
  options and terminal
- Add tests for separator suppression rules

src/options.go

@@ -3699,7 +3699,7 @@ func (opts *Options) noSeparatorLine() bool {
 	// NOTE: This does not know that the default separator can be suppressed at
 	// runtime (see Terminal.separatorLength), so the minimum heights derived
 	// from it can be one line taller than strictly necessary.
-	sep := opts.Separator == nil && !opts.InputBorderShape.Visible() || opts.Separator != nil && len(*opts.Separator) > 0
+	sep := opts.Separator == nil && !inputBorderFacesList(opts.Layout, opts.InputBorderShape) || opts.Separator != nil && len(*opts.Separator) > 0
 	return noSeparatorLine(opts.InfoStyle, sep)
 }