Cover normalized stack strategy sign branches

Follow-up review asked for direct coverage of positive and negative stack
strategy normalization and called out that stacked-by-dimension cache keys are
not number-only. The tests now lock mixed-sign behavior for both strategies, and
the total-cache key type is explicit about accepting string or number values.

Constraint: Do not modify dist; release build artifacts are generated during npm publish
Rejected: Rely on samesign/all tests for positive and negative branches | leaves dedicated strategy branches unprotected
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep normalized stack cache keys aligned with ParsedValue ordinal raw values when changing dimension handling
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 src/processor/dataStack.ts test/ut/spec/series/lineStack.test.ts
Not-tested: Browser visual screenshot against a fresh source bundle; avoided regenerating dist per repo rule