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-attach.test.ts

@@ -18,18 +18,16 @@
  */
 import { beforeEach, describe, expect, it, vi } from 'vitest';
 import {
-  FakeEl,
   FakeTile,
   fetchSpy,
   makeGridApp,
   resetGridHarness,
-  section,
   windowStub,
   type GridApp,
+  tileEl,
 } from './mocks/tile-grid-vm.js';
 
 const IDS = ['s-a', 's-b', 's-c'];
-const tileEl = (id: string) => section.children.find((el) => el.dataset.sessionId === id) as FakeEl;
 const overlayOf = (id: string) => tileEl(id).children[1].children.find((c) => c.className === 'tile-attach') ?? null;
 const visible = (id: string) => !!overlayOf(id) && !overlayOf(id)!.hidden;
 const textOf = (id: string) => overlayOf(id)!.children[0].textContent;