Make heatmap empty cells reviewable in browser

The PR already had unit coverage for empty heatmap cells masking splitArea bands. This adds a focused HTML regression page using the project testHelper harness so reviewers can see and assert the browser behavior directly.

Constraint: Reviewer feedback expects browser-level HTML cases for visual regressions

Confidence: high

Scope-risk: narrow

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

Tested: Browser loaded test/heatmap-empty-cell-splitArea.html after dev-fast bundle and showed checked: Pass