test: add missing coverage (#19)

* 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.

* test: add second batch of missing test coverage

Problem: six more gaps from the audit remained after the first batch —
archive persistence verification, diff modified-on-rename, parse_buffer
inline cat:/due: token parsing, and store.update immutability invariants.

Solution: add six it() blocks across archive_spec, diff_spec, and
store_spec: archive unload/reload persistence check, modified timestamp
updated on description change, inline cat: overrides header category,
inline due: token parsed from buffer line, id/entry fields immutable
under store.update, and end timestamp not overwritten on second
completion.

spec/diff_spec.lua

@@ -57,6 +57,28 @@ describe('diff', function()
       assert.is_nil(result[2].id)
       assert.are.equal('New task here', result[2].description)
     end)
+
+    it('inline cat: token overrides header category', function()
+      local lines = {
+        'Inbox',
+        '/1/  Buy milk cat:Work',
+      }
+      local result = diff.parse_buffer(lines)
+      assert.are.equal(2, #result)
+      assert.are.equal('task', result[2].type)
+      assert.are.equal('Work', result[2].category)
+    end)
+
+    it('inline due: token is parsed', function()
+      local lines = {
+        'Inbox',
+        '/1/  Buy milk due:2026-03-15',
+      }
+      local result = diff.parse_buffer(lines)
+      assert.are.equal(2, #result)
+      assert.are.equal('task', result[2].type)
+      assert.are.equal('2026-03-15', result[2].due)
+    end)
   end)
 
   describe('apply', function()
@@ -107,6 +129,22 @@ describe('diff', function()
       assert.are.equal('Renamed', task.description)
     end)
 
+    it('updates modified when description is renamed', function()
+      local t = store.add({ description = 'Original', category = 'Inbox' })
+      t.modified = '2020-01-01T00:00:00Z'
+      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)
+      assert.is_not.equal('2020-01-01T00:00:00Z', task.modified)
+    end)
+
     it('handles duplicate ids as copies', function()
       store.add({ description = 'Original' })
       store.save()