feat: add screencast fps option

[mturac]

Addresses #2291.

Summary

• add an --experimental-screencast-fps option for screencast recording
• validate the value as a positive integer and imply --experimental-screencast
• pass the configured FPS through to Puppeteer's screencast() call
• update generated option docs and flag telemetry metadata

Testing

• npm run format
• npm run test tests/tools/screencast.test.ts
• npm run build

[mturac]

Validation refreshed on the current head:

git diff --check origin/main...HEAD
npm run test -- tests/tools/screencast.test.ts tests/cli.test.ts
npm run check-format

All three passed locally.

[mturac]

Rebased this branch onto the current main and re-ran validation.

Validation:

• git diff --check origin/main...HEAD
• npm run test -- tests/tools/screencast.test.ts tests/cli.test.ts
• npm run check-format