fix(cache): replace stale M._cache field with get_raw_cache accessor

Problem: M._cache = cache_data captured the initial empty table reference
at module load time. After M.load() reassigns cache_data to the decoded
JSON, M._cache is permanently stale and returns the wrong table.

Solution: remove the field assignment and expose get_raw_cache() which
closes over cache_data and always returns the current table.

lua/cp/cache.lua

@@ -346,6 +346,8 @@ function M.get_data_pretty()
   return vim.inspect(cache_data)
 end
 
-M._cache = cache_data
+function M.get_raw_cache()
+  return cache_data
+end
 
 return M