Show single-root centered tree expansion in the visual case

The tree center layout needs a visual case that makes the intended single-root behavior explicit. The existing case now remains as the explicit side example, and a second chart shows one root splitting first-level branches automatically to both sides.

Constraint: Keep the evidence in the existing tree HTML test case rather than adding another page.
Rejected: Add a second standalone HTML file | one page makes explicit-side and automatic-side behavior easier to compare in the same screenshot.
Confidence: high
Scope-risk: narrow
Reversibility: clean
Tested: git diff --check -- test/tree-center.html
Tested: npx jest --config test/ut/jest.config.cjs --coverage=false test/ut/spec/series/tree.test.ts --runInBand
Tested: npx eslint src/chart/tree/TreeSeries.ts src/chart/tree/treeLayout.ts src/chart/tree/TreeView.ts test/ut/spec/series/tree.test.ts
Tested: Chrome headless screenshot saved at .codex-artifacts/tree-center-single-root.png
Not-tested: Full visual regression suite