fix(fzf): prefer bg-transform, fall back to transform on older fzf

Detect fzf version once at shell startup inside fzf_key_bindings.
Use bg-transform (non-blocking) on fzf >= 0.62 and the synchronous
transform on older builds (e.g. 0.60 devel which lacks bg-transform).
The chosen action is captured into fzf-history-widget via fish's
--inherit-variable so the check runs once, not on every Ctrl+R press.

integrations/fzf.fish

@@ -31,6 +31,14 @@ function fzf_key_bindings
     return 1
   end
 
+  # bg-transform runs the preview toggle in a background thread (non-blocking).
+  # It was added after fzf 0.60; fall back to the synchronous transform on older builds.
+  set -l _fzf_ver (fzf --version | string match -r '^(\d+)\.(\d+)')
+  set -l _fzf_transform_action transform
+  if test -n "$_fzf_ver[3]"; and test "$_fzf_ver[3]" -ge 62
+    set _fzf_transform_action bg-transform
+  end
+
 #----BEGIN INCLUDE common.fish
 # NOTE: Do not directly edit this section, which is copied from "common.fish".
 # To modify it, one can edit "common.fish" and run "./update.sh" to apply
@@ -177,7 +185,7 @@ function fzf_key_bindings
     commandline -f repaint
   end
 
-  function fzf-history-widget -d "Show command history"
+  function fzf-history-widget --inherit-variable _fzf_transform_action -d "Show command history"
     set -l -- command_line (commandline)
     set -l -- current_line (commandline -L)
     set -l -- total_lines (count $command_line)
@@ -206,7 +214,7 @@ function fzf_key_bindings
 
       # Prepend the options to allow user customizations
       set -p -- FZF_DEFAULT_OPTS \
-        '--bind="focus,resize:transform:if test \\"$FZF_COLUMNS\\" -gt 100 -a \\\\( \\"$FZF_SELECT_COUNT\\" -gt 0 -o \\\\( -z \\"$FZF_WRAP\\" -a (string length -- {}) -gt (math $FZF_COLUMNS - 4) \\\\) -o (string collect -- {2..} | fish_indent | count) -gt 1 \\\\); echo show-preview; else echo hide-preview; end"' \
+        '--bind="focus,resize:'$_fzf_transform_action':if test \\"$FZF_COLUMNS\\" -gt 100 -a \\\\( \\"$FZF_SELECT_COUNT\\" -gt 0 -o \\\\( -z \\"$FZF_WRAP\\" -a (string length -- {}) -gt (math $FZF_COLUMNS - 4) \\\\) -o (string collect -- {2..} | fish_indent | count) -gt 1 \\\\); echo show-preview; else echo hide-preview; end"' \
         '--preview="string collect -- (test \\"$FZF_SELECT_COUNT\\" -gt 0; and string collect -- {+2..}) \\"\\n# \\"'$date_cmd' {2..} | fish_indent --ansi"' \
         '--preview-window="right,50%,wrap-word,follow,info,hidden"'
     end