Enable normalized stacked area charts

Line stack values can now opt into stackNormalize to render 100% stacked area charts while preserving the default stacked line behavior. The stack processor normalizes only all-line stack groups and keeps existing stack strategy and stack order semantics. Unit coverage and an HTML visual case lock the behavior, and AGENTS records that dist output is generated during npm publish.

Constraint: dist files are generated during npm publish and must not be manually edited
Rejected: Modify dist or generated declaration output directly | generated artifacts should come from the release flow
Confidence: high
Scope-risk: moderate
Reversibility: clean
Tested: npx jest --config test/ut/jest.config.cjs --coverage=false test/ut/spec/series/lineStack.test.ts
Tested: npm run checktype
Tested: npx eslint src/processor/dataStack.ts src/chart/line/LineSeries.ts test/ut/spec/series/lineStack.test.ts
Tested: git diff --check on changed source, HTML, test, and AGENTS files
Not-tested: npm run test:dts:fast; existing NodeNext declaration errors block the broader d.ts suite before this option is exercised