Spike: run identity under multi-process pytest #297

Part of #277

Unverified whether pytest-xdist workers each generate their own run
id and therefore read as separate runs, which is the known gap for multi-process
Selenium and Nightwatch runs on the JS side.

Acceptance criteria

A written finding with a measurement, and either a fix or
a documented limitation in the README.

Epic B: Cross-language capture architecture

Stop paying for capture logic once per language. Python re-implements roughly
1,206 lines that are not language-specific, and Java would repeat it. Decide and
execute the levers.

Label: capture-architecture

Size: S. Depends on #287.