feat(ui): render the imagery split dividers from a store

createImagerySplit keeps the divider's value, drag, keyboard, CSS position
and scene split, and publishes its line to the imagery split store.
ImagerySplitView, in the scene chrome, teleports each line to the body, so
recent imagery and the Bhote Koshi event no longer build divider DOM.

src/data/bhoteKoshiEvent.test.mjs

@@ -3,6 +3,8 @@ import { readFile } from 'node:fs/promises';
 import test from 'node:test';
 import { claimPointer, releasePointer } from './inputOwnership.js';
 import * as Cesium from 'cesium';
+import { useImagerySplitStore } from '../ui/stores/imagerySplit.js';
+import { applicationPinia } from '../ui/vueRuntime.js';
 import { SOURCE_PATH_BEAT_IDS } from './bhoteKoshiShotPaths.js';
 import {
   BHOTE_KOSHI_LAYER_ID,
@@ -36,6 +38,12 @@ import { SceneDirector } from '../scenes/director.js';
 const eventUrl = new URL('../../public/events/bhote-koshi-2026/event.json', import.meta.url);
 const eventModuleUrl = new URL('./bhoteKoshiEvent.js', import.meta.url);
 
+/** The event's divider as the imagery split store holds it, or undefined. */
+const eventSplit = () =>
+  useImagerySplitStore(applicationPinia()).splits.findLast(
+    (split) => split.view.id === 'bhote-koshi-split-line',
+  );
+
 class FakeElement {
   constructor(tagName = 'div') {
     this.tagName = tagName;
@@ -840,6 +848,7 @@ test('Bhote Koshi disable preserves an operator-selected map and tears down ever
   const originalDocument = globalThis.document;
   const originalWindow = globalThis.window;
   const { body, removedProperties } = installEventDom();
+  assert.equal(eventSplit(), undefined, 'no divider is left from an earlier test');
   const clearedTimeouts = [];
   globalThis.window = {
     setTimeout: () => 71,
@@ -879,7 +888,8 @@ test('Bhote Koshi disable preserves an operator-selected map and tears down ever
     assert.equal(await layer.enable(viewer, { origin: 'scene' }), true);
     assert.equal(viewer._test.imagery.length, 2);
     assert.equal(viewer._test.dataSources.length, 1);
-    assert.equal(body.children.length, 2);
+    assert.equal(body.children.length, 1);
+    assert.ok(eventSplit());
 
     activeMap = 'operator-choice';
     generation += 1;
@@ -889,6 +899,7 @@ test('Bhote Koshi disable preserves an operator-selected map and tears down ever
     assert.equal(viewer._test.removedImagery.length, 2);
     assert.equal(viewer._test.removedDataSources.length, 1);
     assert.ok(body.children.every((element) => element.removed));
+    assert.equal(eventSplit(), undefined);
     assert.equal(viewer.scene.splitPosition, 0.37);
     assert.deepEqual(clearedTimeouts, [71]);
     assert.ok(overlayCalls.some(([kind]) => kind === 'clear'));
@@ -1075,6 +1086,7 @@ test('Bhote Koshi scene beats preserve photoreal and never take camera ownership
   const originalRequestAnimationFrame = globalThis.requestAnimationFrame;
   const originalCancelAnimationFrame = globalThis.cancelAnimationFrame;
   const { body } = installEventDom();
+  assert.equal(eventSplit(), undefined, 'no divider is left from an earlier test');
   const timers = [];
   const openedUrls = [];
   const cancelledFrames = [];
@@ -1183,9 +1195,8 @@ test('Bhote Koshi scene beats preserve photoreal and never take camera ownership
     assert.ok(viewer._test.imagery.every((imageryLayer) => imageryLayer.show === false));
     const panel = body.children.find((element) => element.tagName === 'aside');
     assert.ok(panel);
-    const splitLine = body.children.find((element) => element.id === 'bhote-koshi-split-line');
-    assert.ok(splitLine);
-    assert.equal(splitLine.hidden, true);
+    assert.ok(eventSplit());
+    assert.equal(eventSplit().view.hidden, true);
     assert.equal(panel.querySelector('[data-role="imagery-comparison"]').hidden, true);
     assert.match(panel.querySelector('[data-role="caveat"]').textContent, /SCHEMATIC CORRIDOR/i);
     assert.doesNotMatch(panel.querySelector('[data-role="caveat"]').textContent, /historical/i);
@@ -1240,30 +1251,27 @@ test('Bhote Koshi scene beats preserve photoreal and never take camera ownership
       beatId: 'timure-cluster',
       sceneControls: { imageryComparison: true },
     }, { origin: 'scene' });
-    assert.equal(splitLine.hidden, false);
+    assert.equal(eventSplit().view.hidden, false);
     assert.equal(panel.querySelector('[data-role="imagery-comparison"]').hidden, false);
     assert.ok(viewer._test.imagery.every((imageryLayer) => imageryLayer.show === true));
-    const splitHandle = splitLine.children.find(
-      (element) => element.className === 'bhote-koshi-split-handle',
-    );
-    assert.ok(splitHandle);
-    assert.equal(splitHandle.textContent, '↔');
-    assert.equal(splitHandle.getAttribute('aria-valuenow'), '62');
-    splitHandle.dispatch('pointerdown', {
+    assert.equal(eventSplit().view.handleClass, 'bhote-koshi-split-handle');
+    assert.equal(eventSplit().view.valueNow, 62);
+    const splitHandle = eventSplit().controls;
+    splitHandle.pointerdown({
       button: 0,
       clientX: 250,
       pointerId: 4,
       preventDefault() {},
     });
-    splitHandle.dispatch('pointermove', {
+    splitHandle.pointermove({
       clientX: 750,
       pointerId: 4,
       preventDefault() {},
     });
-    splitHandle.dispatch('pointerup', { pointerId: 4 });
+    splitHandle.pointerup({ pointerId: 4 });
     assert.equal(viewer.scene.splitPosition, 0.75);
-    assert.equal(splitHandle.getAttribute('aria-valuenow'), '75');
-    splitHandle.dispatch('keydown', {
+    assert.equal(eventSplit().view.valueNow, 75);
+    splitHandle.keydown({
       key: 'ArrowLeft',
       shiftKey: true,
       preventDefault() {},
@@ -1276,7 +1284,7 @@ test('Bhote Koshi scene beats preserve photoreal and never take camera ownership
       split: 0.62,
       sceneControls: {},
     }, { origin: 'scene' });
-    assert.equal(splitLine.hidden, true);
+    assert.equal(eventSplit().view.hidden, true);
     assert.ok(viewer._test.imagery.every((imageryLayer) => imageryLayer.show === false));
     assert.equal(viewer.scene.splitPosition, 0.62);
     assert.equal(overlayVisibility.at(-1).visible, false);