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/diff_spec.lua

@@ -0,0 +1,139 @@
+require('spec.helpers')
+
+local config = require('todo.config')
+local store = require('todo.store')
+
+describe('diff', function()
+  local tmpdir
+  local diff = require('todo.diff')
+
+  before_each(function()
+    tmpdir = vim.fn.tempname()
+    vim.fn.mkdir(tmpdir, 'p')
+    vim.g.todo = { data_path = tmpdir .. '/tasks.json' }
+    config.reset()
+    store.unload()
+    store.load()
+  end)
+
+  after_each(function()
+    vim.fn.delete(tmpdir, 'rf')
+    vim.g.todo = nil
+    config.reset()
+  end)
+
+  describe('parse_buffer', function()
+    it('parses headers and tasks', function()
+      local lines = {
+        'School',
+        '/1/  Do homework',
+        '/2/  ! Read chapter 5',
+        '',
+        'Errands',
+        '/3/  Buy groceries',
+      }
+      local result = diff.parse_buffer(lines)
+      assert.are.equal(6, #result)
+      assert.are.equal('header', result[1].type)
+      assert.are.equal('School', result[1].category)
+      assert.are.equal('task', result[2].type)
+      assert.are.equal(1, result[2].id)
+      assert.are.equal('Do homework', result[2].description)
+      assert.are.equal('School', result[2].category)
+      assert.are.equal('task', result[3].type)
+      assert.are.equal(1, result[3].priority)
+      assert.are.equal('blank', result[4].type)
+      assert.are.equal('Errands', result[6].category)
+    end)
+
+    it('handles new tasks without ids', function()
+      local lines = {
+        'Inbox',
+        '  New task here',
+      }
+      local result = diff.parse_buffer(lines)
+      assert.are.equal(2, #result)
+      assert.are.equal('task', result[2].type)
+      assert.is_nil(result[2].id)
+      assert.are.equal('New task here', result[2].description)
+    end)
+  end)
+
+  describe('apply', function()
+    it('creates new tasks from buffer lines', function()
+      local lines = {
+        'Inbox',
+        '  First task',
+        '  Second task',
+      }
+      diff.apply(lines)
+      store.unload()
+      store.load()
+      local tasks = store.active_tasks()
+      assert.are.equal(2, #tasks)
+      assert.are.equal('First task', tasks[1].description)
+      assert.are.equal('Second task', tasks[2].description)
+    end)
+
+    it('deletes tasks removed from buffer', function()
+      store.add({ description = 'Keep me' })
+      store.add({ description = 'Delete me' })
+      store.save()
+      local lines = {
+        'Inbox',
+        '/1/  Keep me',
+      }
+      diff.apply(lines)
+      store.unload()
+      store.load()
+      local active = store.active_tasks()
+      assert.are.equal(1, #active)
+      assert.are.equal('Keep me', active[1].description)
+      local deleted = store.get(2)
+      assert.are.equal('deleted', deleted.status)
+    end)
+
+    it('updates modified tasks', function()
+      store.add({ description = 'Original' })
+      store.save()
+      local lines = {
+        'Inbox',
+        '/1/  Renamed',
+      }
+      diff.apply(lines)
+      store.unload()
+      store.load()
+      local task = store.get(1)
+      assert.are.equal('Renamed', task.description)
+    end)
+
+    it('handles duplicate ids as copies', function()
+      store.add({ description = 'Original' })
+      store.save()
+      local lines = {
+        'Inbox',
+        '/1/  Original',
+        '/1/  Copy of original',
+      }
+      diff.apply(lines)
+      store.unload()
+      store.load()
+      local tasks = store.active_tasks()
+      assert.are.equal(2, #tasks)
+    end)
+
+    it('moves tasks between categories', function()
+      store.add({ description = 'Moving task', category = 'Inbox' })
+      store.save()
+      local lines = {
+        'Work',
+        '/1/  Moving task',
+      }
+      diff.apply(lines)
+      store.unload()
+      store.load()
+      local task = store.get(1)
+      assert.are.equal('Work', task.category)
+    end)
+  end)
+end)