feat: add device position undo history

Issue: #74
User-Visible: yes

test/device-position-history.test.mjs

@@ -0,0 +1,55 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import {
+  applyDevicePlacement,
+  devicePlacement,
+  sameDevicePlacement,
+} from '../test-build/device-position-history.js';
+
+test('updates only placement and preserves zero and future sibling fields', () => {
+  const original = {
+    device: { s: 'floor-1', x: 0.1, y: 0.2, k: 0, future: { pinned: true } },
+  };
+  const next = applyDevicePlacement(original, 'device', {
+    s: 'floor-2', x: 0.75, y: 0.5,
+  });
+
+  assert.deepEqual(next.device, {
+    s: 'floor-2', x: 0.75, y: 0.5, k: 0, future: { pinned: true },
+  });
+  assert.deepEqual(original.device, {
+    s: 'floor-1', x: 0.1, y: 0.2, k: 0, future: { pinned: true },
+  });
+  assert.notEqual(next, original);
+  assert.notEqual(next.device, original.device);
+});
+
+test('legacy placement removes a newer space field without touching siblings', () => {
+  const next = applyDevicePlacement({
+    device: { s: 'floor-1', x: 0.1, y: 0.2, k: 3 },
+  }, 'device', { x: 120, y: 240 });
+  assert.deepEqual(next.device, { x: 120, y: 240, k: 3 });
+  assert.deepEqual(devicePlacement(next, 'device'), { x: 120, y: 240 });
+});
+
+test('null restores auto-position by removing the complete persisted record', () => {
+  const original = {
+    device: { s: 'floor-1', x: 0.1, y: 0.2, k: 0, future: 'value' },
+    other: { s: 'floor-1', x: 0.3, y: 0.4 },
+  };
+  const next = applyDevicePlacement(original, 'device', null);
+  assert.equal('device' in next, false);
+  assert.deepEqual(next.other, original.other);
+  assert.equal(devicePlacement(next, 'device'), null);
+});
+
+test('missing delete is an identity no-op and placement equality includes space', () => {
+  const layout = { other: { x: 1, y: 2 } };
+  assert.equal(applyDevicePlacement(layout, 'missing', null), layout);
+  assert.equal(sameDevicePlacement(null, null), true);
+  assert.equal(sameDevicePlacement(null, { x: 1, y: 2 }), false);
+  assert.equal(sameDevicePlacement({ x: 1, y: 2 }, { x: 1, y: 2 }), true);
+  assert.equal(sameDevicePlacement(
+    { s: 'a', x: 1, y: 2 }, { s: 'b', x: 1, y: 2 },
+  ), false);
+});