Concurrent-navigation flake fixes across CDP and BiDi.

* await_page_ready_after pre_page_id != nil guard
* CDPClient.find_elements_ops nil-id placeholder elimination
* BiDiClient.execute_script "Cannot find context" retry
* Browser.find/2 retries on :stale_reference
* Page-ready state machine + ring buffer in NavigationTimeoutError
* Drop chromedriver references and dead Docker fallback
* Hex package: exclude priv/bidi-server/node_modules

See CHANGELOG.md for details.