Align treemap HTML regression with parent selection

The existing browser case selected the child index while asserting the parent-node upperLabel background behavior. This tightens the HTML regression to exercise the same parent node as the unit test and keeps the reviewer-facing instructions visual.

Constraint: Reviewer feedback expects browser-level HTML cases that match ECharts test conventions

Confidence: high

Scope-risk: narrow

Tested: npm test -- --runInBand test/ut/spec/series/treemap.test.ts

Tested: Browser loaded test/treemap-upperLabel-select.html after dev-fast bundle and showed checked: Pass