Use store loop for sampling extent count

Copilot flagged that SeriesData.each still adds callback binding overhead in the visible-window count path. This keeps the existing scale containment semantics but reads parsed values through the underlying DataStore with a precomputed dimension index.

Constraint: direct min/max checks from scale.getExtent did not preserve the filterMode none regression case

Confidence: high

Scope-risk: narrow

Tested: npm run checktype -- --pretty false

Tested: npm run lint -- --quiet src/processor/dataSample.ts

Tested: git diff --check -- src/processor/dataSample.ts

Tested: test/line-sampling-dataZoom-filterMode.html via Chrome, PASS with store-loop count

Not-tested: Full visual regression suite