Preserve line sampling detail inside dataZoom windows

Line sampling previously used the processed series count as its sampling base. When dataZoom used filterMode none or empty, that count still represented the full dataset, so a narrow zoom could remain downsampled as if the whole dataset were visible.

This bases the sampling rate on points contained by the current base-axis extent while keeping the no-sampling path out of the processor. The regression HTML case compares filter, none, empty, and raw modes around a narrow zoom window.

Constraint: dataZoom filterMode none and empty intentionally keep full SeriesData rather than filtering rows before sampling

Rejected: Change dataZoom filtering semantics for none/empty | would break the documented promise that data is not filtered

Confidence: high

Scope-risk: narrow

Directive: Sampling rate should follow the visible base-axis extent, not only the post-filter SeriesData count

Tested: npm run checktype -- --pretty false

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

Tested: test/line-sampling-dataZoom-filterMode.html via Chrome, PASS with none/empty counts matching raw

Not-tested: Full visual regression suite