Turn g:loaded_fzf into a version marker

Callers cannot detect plugin-side behavior. fzf#exec() reports the version
of the binary, not of this plugin, and everything else here is script-local.
fzf.vim needs to know whether fzf#run is asynchronous in popup mode before
offering a key binding that depends on it.

The value was only ever read through the exists() guard, so raising it from
1 breaks nothing.

plugin/fzf.vim

@@ -24,7 +24,8 @@
 if exists('g:loaded_fzf')
   finish
 endif
-let g:loaded_fzf = 1
+" Bumped when the plugin gains behavior a caller may need to detect
+let g:loaded_fzf = 20260821
 
 let s:is_win = has('win32') || has('win64')
 if s:is_win && &shellslash