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-auto-join.test.ts

@@ -18,7 +18,7 @@
 import { readFileSync } from 'node:fs';
 import { resolve } from 'node:path';
 import { beforeEach, describe, expect, it, vi } from 'vitest';
-import { FakeEl, FakeTile, makeGridApp, resetGridHarness, section, type GridApp } from './mocks/tile-grid-vm.js';
+import { FakeTile, makeGridApp, resetGridHarness, section, type GridApp, tileEl } from './mocks/tile-grid-vm.js';
 
 const IDS = ['s-a', 's-b'];
 const addSession = (app: GridApp, id: string, workingDir = '/w') =>
@@ -52,7 +52,7 @@ describe('Run from this tab', () => {
     app.openTileGrid(IDS);
     addSession(app, 's-new');
     app._joinTileGridFromRun('s-new');
-    const tile = section.children.find((el) => el.dataset.sessionId === 's-new') as FakeEl;
+    const tile = tileEl('s-new');
     const overlay = tile.children[1].children.find((c) => c.className === 'tile-attach');
     expect(!overlay || overlay.hidden).toBe(true);
   });