feat(orgchart): integrate chart-only viewport and scoped guard fixes

Add initial fit and direct wheel zoom with preserved default presentation. Integrate five scoped GuardFix findings and AI generation preselection policy. Exclude Cell experiments and retain five historical guard failures.

examples/org-chart-chart-only/vanilla.ts

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+import { createOrgChart, JsonOrgChartDataSource } from '@hm-design/org-chart';
+import engineUrl from '@hm-design/org-chart/engine.css?url';
+import coreUrl from '@hm-design/org-chart/style.css?url';
+import { datasets, readQuery } from './data';
+import './chart-only.css';
+
+const { data, layout, embed } = readQuery();
+if (embed) document.body.classList.add('embed');
+const chart = createOrgChart({
+  container: document.getElementById('chart')!,
+  dataSource: new JsonOrgChartDataSource(datasets[data]!()),
+  stylesheetUrls: { engine: engineUrl, core: coreUrl },
+  displayMode: 'readOnly',
+  layoutMode: 'auto',
+  autoLayout: { type: layout },
+  // Chart Only implies initialView "fit" and wheelZoom "direct".
+  presentationMode: 'chart-only',
+  // Regression fixture: Fit below the manual zoom floor, without huge data.
+  zoom: new URLSearchParams(location.search).has('verify-min') ? { min: 0.8 } : undefined,
+});
+void chart.mount();
+// Verification seam only: replace the whole dataset through the public handle.
+Object.assign(window, { orgChart: chart, replaceData: (key: string) => chart.update(datasets[key]!()) });