test: add store, parse, and diff specs

Problem: need test coverage for core data operations, inline
metadata parsing, and buffer diff algorithm.

Solution: add busted specs for store CRUD, round-trip
preservation, parse body/command_add with configurable date
syntax, and diff create/delete/update/copy/move operations.

spec/helpers.lua

@@ -0,0 +1,38 @@
+local plugin_dir = vim.fn.getcwd()
+vim.opt.runtimepath:prepend(plugin_dir)
+vim.opt.packpath = {}
+
+vim.cmd('filetype on')
+
+local M = {}
+
+---@param lines? string[]
+---@return integer
+function M.create_buffer(lines)
+  local bufnr = vim.api.nvim_create_buf(false, true)
+  vim.api.nvim_buf_set_lines(bufnr, 0, -1, false, lines or {})
+  return bufnr
+end
+
+---@param bufnr integer
+function M.delete_buffer(bufnr)
+  if bufnr and vim.api.nvim_buf_is_valid(bufnr) then
+    vim.api.nvim_buf_delete(bufnr, { force = true })
+  end
+end
+
+---@param bufnr integer
+---@param ns integer
+---@return table[]
+function M.get_extmarks(bufnr, ns)
+  return vim.api.nvim_buf_get_extmarks(bufnr, ns, 0, -1, { details = true })
+end
+
+---@return string
+function M.tmpdir()
+  local dir = vim.fn.tempname()
+  vim.fn.mkdir(dir, 'p')
+  return dir
+end
+
+return M