Make normalized area visual case denser

The normalized stacked area HTML case now uses 100 deterministic data points so the preview better represents a realistic dense area chart while preserving the same top-boundary-at-one verification shape.

Constraint: Visual fixture should stay deterministic for screenshot review
Rejected: Use random data | screenshots would drift between runs
Confidence: high
Scope-risk: narrow
Reversibility: clean
Tested: npx jest --config test/ut/jest.config.cjs --coverage=false test/ut/spec/series/lineStack.test.ts
Tested: git diff --check -- test/area-stack.html
Tested: Browser preview at test/area-stack.html with 100 data points
Not-tested: broader visual regression suite