perf: defer registered plugins cache update to UIEnter

Using vim.schedule alone still impacted startup time. Deferring
update_registered_plugins_cache to UIEnter (like VeryLazy) ensures
the cache is populated after the UI loads, improving perceived
startup performance.

Updated test helper flush_pending() to trigger UIEnter so tests
properly simulate the deferred behavior.

Author: zuqini

lua/zpack/init.lua

@@ -154,7 +154,12 @@ M.setup = function(opts)
 
   local commands = require('zpack.commands')
   commands.setup(config.cmd_prefix)
-  vim.schedule(commands.update_registered_plugins_cache)
+  vim.api.nvim_create_autocmd("UIEnter", {
+    callback = function()
+      vim.schedule(commands.update_registered_plugins_cache)
+    end,
+    once = true,
+  })
 end
 
 ---@deprecated Use setup({ spec = { ... } }) instead

tests/helpers.lua

@@ -275,6 +275,8 @@ function M.wait_for_condition(condition, timeout_ms, interval_ms)
 end
 
 function M.flush_pending()
+  -- Trigger UIEnter to fire autocmds that defer work until after UI loads
+  vim.api.nvim_exec_autocmds("UIEnter", {})
   -- Process all pending vim.schedule callbacks by waiting with a condition that
   -- never returns true. 50ms is sufficient for most deferred operations while
   -- keeping tests fast. The condition returns false to ensure we always wait

tests/lazy_event_test.lua

@@ -118,13 +118,13 @@ return function()
         defaults = { confirm = false },
       })
 
-      helpers.flush_pending()
       local autocmds = vim.api.nvim_get_autocmds({ group = state.lazy_group })
       helpers.assert_not_nil(
         helpers.find_autocmd(autocmds, 'UIEnter'),
         "VeryLazy should create UIEnter autocmd"
       )
 
+      helpers.flush_pending()
       helpers.cleanup_test_env()
     end)