feat(donut-s2): Hover - segment hover fade, direct-label color switch, and legend bidirectional highlight

[madelineluke]

Description

Implements the donut-hover spec: hovering a donut segment fades its siblings, switches the hovered segment's direct-label value to its own categorical color, and syncs highlight state bidirectionally with a paired <Legend>.

• Extends isInteractive so a donut with a SegmentLabel showing value/percent is hover-interactive on its own, with no popover/inspect required. Verified no other mark type declares a segmentLabels field, so this is a no-op everywhere else.
• Adds a SERIES_ID data transform (reusing the existing generic getSeriesIdTransform helper) whenever a donut is interactive or paired with a highlighting Legend - needed for both hover directions with a Legend to actually match rows, since donut's data never had this field before, unlike Line/Bar.
• Both SegmentLabel text marks now fade via the same getMarkOpacity mechanism the arc mark already used, and the value line switches to the hovered segment's own categorical color (the name line stays gray-700, unchanged) - matching the Figma reference exactly.
• Adds legendHighlightSignals to DonutSpecOptions and consumes it on the arc mark, fading non-matching segments when a paired Legend entry is hovered - the reverse direction of the arc's own hover already highlighting the legend generically via userMeta.interactiveMarks.
• Adds Donut/Features/Hover stories: a bare SegmentLabel case (no popover) and a Legend-paired case demonstrating both hover directions.

Related Issue

planning/specs/donut/donut-hover.json (approved)

Motivation and Context

Today a donut with only a direct label configured (no popover/inspect) produces no hover feedback at all, and even where hover exists (via a popover), the direct label's value never reflects which segment is hovered and never fades with its de-emphasized arc. Separately, a donut paired with a <Legend> never highlighted in either direction, unlike Line/Bar, because the generic legendHighlightSignals mechanism was already being spread into donut's options but silently dropped (untyped, unconsumed), and donut's data was missing the SERIES_ID field the legend's generic hover matching relies on.

How Has This Been Tested?

• Unit tests added/updated in markUtils.test.ts, donutSpecBuilder.test.ts, donutUtils.test.ts, and a new hover behavior block in segmentLabelUtils.test.tsx.
• Full vega-spec-builder-s2/react-spectrum-charts-s2 test suite passes (131 suites, 2120 tests).
• yarn lint and yarn tsc --noEmit are clean (no new errors; two pre-existing unrelated failures confirmed present on main too).
• Verified live in Storybook via Playwright, using precise on-pixel hover simulation (not bounding-box centers, which land inside a wedge's hole for thin rings): hovering an arc fades siblings to 0.2 opacity, keeps the hovered value's color, and highlights the matching legend entry; hovering a legend entry fades non-matching arcs. Both directions confirmed working end-to-end.

Screenshots (if appropriate):

See the SimpleDirectLabel and WithLegend stories under React Spectrum Charts 2/Donut/Features/Hover in Storybook.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

🎨 Storybook -> https://opensource.adobe.com/react-spectrum-charts/PR-895

🎨 S2 Storybook -> https://opensource.adobe.com/react-spectrum-charts/PR-895-s2

📚 Docs -> https://opensource.adobe.com/react-spectrum-charts/PR-895-docs/