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,

spec/commands_spec.lua

@@ -21,13 +21,6 @@ describe('commands', function()
       end)
     end)
 
-    it('does not error on :Preview stop', function()
-      require('preview.commands').setup()
-      assert.has_no.errors(function()
-        vim.cmd('Preview stop')
-      end)
-    end)
-
     it('does not error on :Preview status', function()
       require('preview.commands').setup()
       assert.has_no.errors(function()