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/archive_spec.lua

@@ -128,4 +128,13 @@ describe('archive', function()
     assert.is_true(descs['Keep pending'])
     assert.is_true(descs['Keep recent done'])
   end)
+
+  it('persists archived tasks to disk after unload/reload', function()
+    local t = store.add({ description = 'Archived task' })
+    store.update(t.id, { status = 'done', ['end'] = '2020-01-01T00:00:00Z' })
+    pending.archive()
+    store.unload()
+    store.load()
+    assert.are.equal(0, #store.active_tasks())
+  end)
 end)

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()

spec/store_spec.lua

@@ -121,6 +121,27 @@ describe('store', function()
       local updated = store.get(t.id)
       assert.is_not_nil(updated['end'])
     end)
+
+    it('does not overwrite id or entry', function()
+      store.load()
+      local t = store.add({ description = 'Immutable fields' })
+      local original_id = t.id
+      local original_entry = t.entry
+      store.update(t.id, { id = 999, entry = 'x' })
+      local updated = store.get(original_id)
+      assert.are.equal(original_id, updated.id)
+      assert.are.equal(original_entry, updated.entry)
+    end)
+
+    it('does not overwrite end on second completion', function()
+      store.load()
+      local t = store.add({ description = 'Complete twice' })
+      store.update(t.id, { status = 'done', ['end'] = '2026-01-15T10:00:00Z' })
+      local first_end = store.get(t.id)['end']
+      store.update(t.id, { status = 'done' })
+      local task = store.get(t.id)
+      assert.are.equal(first_end, task['end'])
+    end)
   end)
 
   describe('delete', function()