Skip sampling work for collapsed axis size

Copilot flagged that a collapsed base-axis pixel extent still triggered the visible-count scan before sampling was skipped by a non-finite rate. This exits before scanning when the calculated sampling size is non-finite or non-positive.

Constraint: zero-size chart containers can occur transiently during layout or resize

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 zero-size guard

Not-tested: Full visual regression suite