feat: expose screencast first frame timestamp

[Sushanth012]

Summary

• capture the first Page.screencastFrame metadata timestamp while starting a screencast
• include the timestamp in the human-readable screencast_start response
• expose the timestamp as structuredContent.firstFrameTimestamp for clients that need precise video annotation alignment
• add regression coverage for the response text and structured content

Verification

• npm run test tests/tools/screencast.test.ts
• npm run typecheck
• npm run check-format

Note: on Windows, npm run check-format runs ESLint and Prettier successfully, then fails because the package script's trailing semicolon is passed to Prettier as a ".;" file pattern.

Closes #2216

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