feat(player): seed editor region handoff state (#762)

Signed-off-by: ChrisBeWithYou <christian.a.cowan@gmail.com>

tests/js/editor_pending_view.test.js

@@ -0,0 +1,47 @@
+'use strict';
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+
+const src = fs.readFileSync(path.join(__dirname, '..', '..', 'static', 'app.js'), 'utf8');
+const m = src.match(/\/\* @pure:editor-pending-view:start \*\/[\s\S]*?\/\* @pure:editor-pending-view:end \*\//);
+if (!m) throw new Error('pending-view helper block not found');
+
+const api = new Function('"use strict";' + m[0] + '\nreturn { _buildEditorPendingViewPure };')();
+
+test('edit-region handoff defaults cursor to region start and marks return path', () => {
+    const out = api._buildEditorPendingViewPure('song.sloppak', 2, { a: 12.5, b: 20 }, { returnToHighway: true });
+    assert.deepStrictEqual(out, {
+        filename: 'song.sloppak',
+        arrangement: 2,
+        barSel: { startTime: 12.5, endTime: 20 },
+        returnToHighway: true,
+        cursorTime: 12.5,
+    });
+});
+
+test('return-trip handoff preserves explicit viewport state', () => {
+    const out = api._buildEditorPendingViewPure('song.sloppak', 1, { a: 8, b: 14 }, {
+        scrollX: -4,
+        zoom: 160,
+        cursorTime: 9.25,
+    });
+    assert.deepStrictEqual(out, {
+        filename: 'song.sloppak',
+        arrangement: 1,
+        barSel: { startTime: 8, endTime: 14 },
+        cursorTime: 9.25,
+        scrollX: 0,
+        zoom: 160,
+    });
+});
+
+test('missing region still produces a stable pending view shell', () => {
+    const out = api._buildEditorPendingViewPure('song.sloppak', -1, null, {});
+    assert.deepStrictEqual(out, {
+        filename: 'song.sloppak',
+        arrangement: 0,
+        barSel: null,
+    });
+});