refactor(commands): remove stop subcommand

Problem: :Preview stop had a subtle distinction from toggle-off (kill
process but keep autocmd) that nobody reaches for deliberately from
the command line.

Solution: remove stop from the command dispatch table. The Lua API
require('preview').stop() remains as a programmatic escape hatch.

lua/preview/commands.lua

@@ -4,9 +4,6 @@ local handlers = {
   compile = function()
     require('preview').compile()
   end,
-  stop = function()
-    require('preview').stop()
-  end,
   clean = function()
     require('preview').clean()
   end,