test: add top-priority missing test coverage

Problem: several critical code paths had zero test coverage —
parse.resolve_date (relative date resolution), store.snapshot
(foundation of the undo stack), and the diff.apply invariant that
unchanged tasks do not get their modified timestamp bumped. The
diff.apply due/priority clearing paths were also untested.

Solution: add six targeted test blocks across parse_spec, store_spec,
and diff_spec: resolve_date happy/failure paths, parse.body with
relative due tokens, snapshot copy-semantics and deleted-task
exclusion, diff unchanged-modified invariant, due cleared on removal,
priority cleared on ! removal.

spec/diff_spec.lua

@@ -135,5 +135,51 @@ describe('diff', function()
       local task = store.get(1)
       assert.are.equal('Work', task.category)
     end)
+
+    it('does not update modified when task is unchanged', function()
+      store.add({ description = 'Stable task', category = 'Inbox' })
+      store.save()
+      local lines = {
+        'Inbox',
+        '/1/  Stable task',
+      }
+      diff.apply(lines)
+      store.unload()
+      store.load()
+      local modified_after_first = store.get(1).modified
+      diff.apply(lines)
+      store.unload()
+      store.load()
+      local task = store.get(1)
+      assert.are.equal(modified_after_first, task.modified)
+    end)
+
+    it('clears due when removed from buffer line', function()
+      store.add({ description = 'Pay bill', due = '2026-03-15' })
+      store.save()
+      local lines = {
+        'Inbox',
+        '/1/  Pay bill',
+      }
+      diff.apply(lines)
+      store.unload()
+      store.load()
+      local task = store.get(1)
+      assert.is_nil(task.due)
+    end)
+
+    it('clears priority when ! is removed from buffer line', function()
+      store.add({ description = 'Task name', priority = 1 })
+      store.save()
+      local lines = {
+        'Inbox',
+        '/1/  Task name',
+      }
+      diff.apply(lines)
+      store.unload()
+      store.load()
+      local task = store.get(1)
+      assert.are.equal(0, task.priority)
+    end)
   end)
 end)

spec/parse_spec.lua

@@ -87,6 +87,67 @@ describe('parse', function()
       assert.are.equal('Buy milk due:2026-03-15', desc)
       assert.is_nil(meta.due)
     end)
+
+    it('resolves due:today to today date', function()
+      local desc, meta = parse.body('Buy milk due:today')
+      assert.are.equal('Buy milk', desc)
+      assert.are.equal(os.date('%Y-%m-%d'), meta.due)
+    end)
+
+    it('resolves due:+2d to today plus 2 days', function()
+      local today = os.date('*t') --[[@as osdate]]
+      local expected = os.date('%Y-%m-%d', os.time({ year = today.year, month = today.month, day = today.day + 2 }))
+      local desc, meta = parse.body('Task due:+2d')
+      assert.are.equal('Task', desc)
+      assert.are.equal(expected, meta.due)
+    end)
+
+    it('leaves unresolvable due token in description', function()
+      local desc, meta = parse.body('Task due:garbage')
+      assert.is_nil(meta.due)
+      assert.truthy(desc:find('due:garbage', 1, true))
+    end)
+  end)
+
+  describe('parse.resolve_date', function()
+    it("returns today's date for 'today'", function()
+      local result = parse.resolve_date('today')
+      assert.are.equal(os.date('%Y-%m-%d'), result)
+    end)
+
+    it("returns tomorrow's date for 'tomorrow'", function()
+      local expected = os.date('%Y-%m-%d', os.time() + 86400)
+      local result = parse.resolve_date('tomorrow')
+      assert.are.equal(expected, result)
+    end)
+
+    it("returns today + 3 days for '+3d'", function()
+      local today = os.date('*t') --[[@as osdate]]
+      local expected = os.date('%Y-%m-%d', os.time({ year = today.year, month = today.month, day = today.day + 3 }))
+      local result = parse.resolve_date('+3d')
+      assert.are.equal(expected, result)
+    end)
+
+    it("returns today for '+0d'", function()
+      local result = parse.resolve_date('+0d')
+      assert.are.equal(os.date('%Y-%m-%d'), result)
+    end)
+
+    it("returns a future Monday (or today) for 'mon'", function()
+      local result = parse.resolve_date('mon')
+      assert.is_not_nil(result)
+      assert.truthy(result:match('^%d%d%d%d%-%d%d%-%d%d$'))
+    end)
+
+    it("returns nil for garbage input", function()
+      local result = parse.resolve_date('notadate')
+      assert.is_nil(result)
+    end)
+
+    it("returns nil for empty string", function()
+      local result = parse.resolve_date('')
+      assert.is_nil(result)
+    end)
   end)
 
   describe('command_add', function()

spec/store_spec.lua

@@ -186,4 +186,31 @@ describe('store', function()
       assert.are.equal('Active', active[1].description)
     end)
   end)
+
+  describe('snapshot', function()
+    it('returns a table of tasks', function()
+      store.load()
+      store.add({ description = 'Snap one' })
+      store.add({ description = 'Snap two' })
+      local snap = store.snapshot()
+      assert.are.equal(2, #snap)
+    end)
+
+    it('returns a copy that does not affect the store', function()
+      store.load()
+      local t = store.add({ description = 'Original' })
+      local snap = store.snapshot()
+      snap[1].description = 'Mutated'
+      local live = store.get(t.id)
+      assert.are.equal('Original', live.description)
+    end)
+
+    it('excludes deleted tasks', function()
+      store.load()
+      local t = store.add({ description = 'Will be deleted' })
+      store.delete(t.id)
+      local snap = store.snapshot()
+      assert.are.equal(0, #snap)
+    end)
+  end)
 end)