perf(s2): look up hover fractions with an indexed expression function

[c-lamoureux]

Stacked on #990.

What

• New hoverFraction(rows, keyField, key, fallback) expression function. It indexes hoverFractionData rows by key once per data array (a WeakMap cache) and reads each row's value live.
• Line, bar and legend hover opacity use it instead of indexof(pluck(data(...))), which scanned every series for every item on every frame.
• Storybook S2: turns off Storybook 10's built-in highlight feature (features.highlight: false). It runs querySelectorAll('*') + getComputedStyle on every DOM change in the story, which skewed hover benchmarks.
• The line hover dashboard perf story gets a renderer (svg/canvas) arg.

Why

With the CSP-safe interpreter, the pluck/indexof lookup was O(series) per item per frame. The lookup is now O(1) after one index build per data change.

Perf

• Interpreted nodes per lookup: 25 → 19; the per-frame pluck is gone.
• 20-chart line hover dashboard, 4× CPU, 5 runs: 18.5 → 20.0 median chart fps (within noise; a modest gain).

Testing

• Unit tests for hoverFraction (match, fallback, first row wins, live reads).
• A real Vega test (CSP interpreter, ast: true) that applies modify/insert changesets and checks the lookup stays current. It fails if the function caches values instead of rows.
• vega-interpreter added as a devDependency for that test only; no runtime dependency change.
• S1 has no hover-fraction code, so no S1 change.

[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-991/

🎨 S2 Storybook -> https://opensource.adobe.com/react-spectrum-charts/PR-991-s2/

🎨 S2 Dev Storybook -> https://opensource.adobe.com/react-spectrum-charts/PR-991-s2-dev/

📚 Docs -> https://opensource.adobe.com/react-spectrum-charts/PR-991-docs/