Avoid unused normalized stack total caches

Follow-up review noted that the normalized stack total cache still did extra work
by filling both index and dimension maps. The processor now builds each cache
lazily for the stack mode that actually needs it, preserving mixed-mode safety
without doing unused per-point aggregation. Additional line stack tests cover
negative normalized values and the non-default all stack strategy.

Constraint: Do not modify dist; release build artifacts are generated during npm publish
Rejected: Build both total maps eagerly | wastes work and allocations for the common single-mode stack group
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep stack total cache construction lazy if future stack modes add more denominator keys
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