feat(buffer): persist fold state across sessions (#94)

Problem: folded category headers are lost when Neovim exits because
`_fold_state` only lives in memory. Users must re-fold categories
every session.

Solution: store folded category names in the JSON data file as a
top-level `folded_categories` field. On first render, `restore_folds`
seeds from the store instead of the empty in-memory state. Folds are
persisted on `M.close()` and `VimLeavePre`.

lua/pending/init.lua

@@ -268,6 +268,18 @@ function M._setup_autocmds(bufnr)
       end
     end,
   })
+  vim.api.nvim_create_autocmd('VimLeavePre', {
+    group = group,
+    callback = function()
+      local bnr = buffer.bufnr()
+      log.debug(('VimLeavePre: bufnr=%s valid=%s'):format(
+        tostring(bnr), tostring(bnr and vim.api.nvim_buf_is_valid(bnr))))
+      if bnr and vim.api.nvim_buf_is_valid(bnr) then
+        buffer.persist_folds()
+        get_store():save()
+      end
+    end,
+  })
 end
 
 ---@param bufnr integer