CDP push-mode screencast and performance capture #292

Part of #277

Video is one screenshot per command on the main thread, which is
deliberate because a Selenium session is not thread-safe and a poll thread races
the test's own commands. Chrome's Page.startScreencast would give a real frame
stream, and performance entries are not captured at all.

Acceptance criteria

Chrome uses a push-mode screencast when available, with
the per-command path as the fallback for every other browser. Performance entries
reach the dashboard.

Size: M.