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)