test(tiles): the grid harness finds a tile's element, and serves the pure helpers

Five tile-grid tests defined the same `tileEl(id)` lookup and three more
inlined it; the harness (test/mocks/tile-grid-vm.ts) now exports it and
they import it. tile-grid-open-set built a second vm context just to read
constants.js, although the harness it already imports has loaded the same
file: it reads windowStub.CodemanTileGrid instead.

No assertion changed. Mutation-checked: tiles without their
data-session-id fail 34 tests across the seven files, and a broken
tileGridOpenSet fails the open-set cases.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/tile-grid-select-branch.test.ts

@@ -26,8 +26,8 @@ import {
   localStore,
   makeGridApp,
   resetGridHarness,
-  section,
   type GridApp,
+  tileEl,
 } from './mocks/tile-grid-vm.js';
 
 const IDS = ['s-a', 's-b', 's-c'];
@@ -163,7 +163,7 @@ describe('moving focus between tiles', () => {
   it('pressing a tile is a human selection of its session', async () => {
     const app = openGrid();
     app.selectSession = vi.fn();
-    const tileB = section.children.find((el) => el.dataset.sessionId === 's-b');
+    const tileB = tileEl('s-b');
     tileB?.dispatch('pointerdown');
     expect(app.selectSession).toHaveBeenCalledWith('s-b');
   });