perf: async cache initialization and remove deepcopy

Problem: first completion request blocked the UI with a synchronous
vim.system():wait() call, and every subsequent key completion
unnecessarily deep-copied the entire cache.

Solution: use vim.system with an async callback to initialize the cache
without blocking. Queue pending completion requests during loading and
serve them once parsing finishes. Return cached keys directly instead
of deep-copying.

spec/ghostty_spec.lua

@@ -19,24 +19,31 @@ local BASH_COMPLETION = table.concat({
 }, '\n')
 
 local function mock_system()
-  local original = vim.system
+  local original_system = vim.system
+  local original_schedule = vim.schedule
   ---@diagnostic disable-next-line: duplicate-set-field
-  vim.system = function(cmd)
+  vim.system = function(cmd, _, on_exit)
     if cmd[1] == 'ghostty' then
-      return {
-        wait = function()
-          return { stdout = CONFIG_DOCS, code = 0 }
-        end,
-      }
+      local result = { stdout = CONFIG_DOCS, code = 0 }
+      if on_exit then
+        on_exit(result)
+        return {}
+      end
+      return { wait = function() return result end }
     end
-    return {
-      wait = function()
-        return { stdout = '', code = 1 }
-      end,
-    }
+    local result = { stdout = '', code = 1 }
+    if on_exit then
+      on_exit(result)
+      return {}
+    end
+    return { wait = function() return result end }
+  end
+  vim.schedule = function(fn)
+    fn()
   end
   return function()
-    vim.system = original
+    vim.system = original_system
+    vim.schedule = original_schedule
   end
 end