Add resetRowNumbers grouping flag to Grid

[mstijak]

Summary

Adds a per-grouping-level resetRowNumbers flag to Grid. When set on a grouping level, the automatic cxe-grid-row-number column restarts numbering from 1 at the beginning of each group instead of counting continuously across the whole grid.

grouping={[
   { key: { continent: { bind: "$record.continent" } }, showCaption: true, resetRowNumbers: true },
]}
columns={[
   { header: "#", defaultWidth: 50, children: <div class="cxe-grid-row-number" /> },
   // ...
]}

Implementation

The row-number column is a pure CSS counter: the grid container does counter-reset: cx-row-number <start> and each .cxe-grid-row-number::after does counter-increment. The container reset is document-global isolation — without it, two grids on the same page would continue each other's numbering.

The non-obvious part: a counter-reset placed on the group caption row creates a nested counter that is shadowed by the container's reset, so the following data rows ignore it (numbering stays continuous). Using counter-set on the caption instead updates the existing in-scope counter in place, which correctly resets per group while keeping the container reset for cross-grid isolation. Verified in the browser across multiple groups and against a preceding row-numbered grid.

Because the reset rides on the caption row, the grouping level must render a caption (showCaption or caption) for it to take effect — documented.

Changes

• packages/cx/src/widgets/grid/Grid.tsx — resetRowNumbers?: boolean on GridGroupingConfig; renderGroupHeader adds a reset-row-numbers state mod to the group-caption <tbody>.
• packages/cx/src/widgets/grid/Grid.scss — caption rule counter-set: cx-row-number 0.
• homedocs/ — new "Per-Group Row Numbers" docs section + example, config-table row, and changelog entry.

Test plan

• Verified live in the docs example: Asia 1‑2, Europe 1‑3, North America 1‑3.
• Confirmed cross-grid isolation still holds (group reset does not leak the counter from a preceding grid).