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/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)