perf(s2): compute hover deemphasis opacity once per series

[c-lamoureux]

What

• hoverFractionData gets a deemphasisOpacity formula field, so each series' faded opacity is computed once per animation row instead of once per data point.
• Line, bar and donut opacity read it via hoverFraction(..., 'deemphasisOpacity'); hoverFraction gains an optional field argument (defaults to fraction).
• No visual change: the formula is the same FADE_FACTOR + (1 - FADE_FACTOR) * ramp(fraction) expression, moved into the data.

Why

With the CSP-safe interpreter, every vertex evaluated the full ramp expression on each hover animation frame. Moving it to one row per series removes that per-vertex math.

Perf

• Opacity interpreter nodes per call: 19 → 9; interpreter nodes per line vertex: 43.5 → 34.6 (−20%).
• Headless dataflow benchmark, line 30 series × 10 points: ~1.0 → 0.85 ms per frame (~15%, consistent across 3 alternating pairs).
• Browser line hover dashboard: no measurable fps change. Hover is render-bound there, so this mainly frees script time.

Testing

• Unit tests for the new field, the hoverFraction field argument, and updated line, bar and point opacity expectations.
• S2 only; S1 has no hover animation code.

[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-1002/

🎨 S2 Storybook -> https://opensource.adobe.com/react-spectrum-charts/PR-1002-s2/

🎨 S2 Dev Storybook -> https://opensource.adobe.com/react-spectrum-charts/PR-1002-s2-dev/

📚 Docs -> https://opensource.adobe.com/react-spectrum-charts/PR-1002-docs/