Epic: Python Selenium adapter for the DevTools dashboard

Bring the DevTools dashboard to Python Selenium as a fourth adapter, over the existing language-neutral { scope, data } JSON contract, with no backend or UI changes. This issue tracks the adapter itself: capture parity with the JavaScript adapters, the features not yet built, release readiness and docs.

Each sub-issue is scoped to one pull request. Progress is tracked by the sub-issue list; the grouping below is the map.

Correctness and visibility #279, #280, #281, #282, #283
Capture parity with the JS adapters #284, #285, #286, #287
Features not yet built #289, #291, #292, #293, #324
Release and hygiene #294, #295, #296, #297, #334, #339

Ordering note: #284 comes before #285 and #286. On the JavaScript side, document-start preload removed the race class those two exist to patch around, so doing them first means writing code that #284 then reduces to a narrow backstop.

Trace mode moved to #330. #288 and #290 are no longer tracked here. Everything
this epic covers is LIVE mode — commands, test tree, console and network over
BiDi, DOM snapshots, assertions, screencast, run controls, Preserve & Rerun — and
all of it is delivered. Trace mode is a separate capability sitting behind the
#298 architecture decision, and keeping it here meant this epic could not close
on work that is finished.

What remains here is release work: #334 (make the Node requirement explicit)
and #294 (PyPI). This epic closes when those land.