This is your first big boi, "from GH issue" design, plan and implement task. We need to try and add sub-interpreter (aka subint) support per the issue, https://github.com/goodboy/tractor/issues/379 Part of this work should include, - modularizing and thus better organizing the `.spawn.*` subpkg by breaking up various backends currently in `spawn._spawn` into separate submods where it makes sense. - add a new `._subint` backend which tries to keep as much of the inter-process-isolation machinery in use as possible but with plans to optimize for localhost only benefits as offered by python's subints where possible. * utilizing localhost-only tpts like UDS, shm-buffers for performant IPC between subactors but also leveraging the benefits from the traditional OS subprocs mem/storage-domain isolation, linux namespaces where possible and as available/permitted by whatever is happening under the hood with how cpython implements subints. * default configuration should encourage state isolation as with subprocs, but explicit public escape hatches to enable rigorously managed shm channels for high performance apps. - all tests should be (able to be) parameterized to use the new `subints` backend and enabled by flag in the harness using the existing `pytest --spawn-backend ` support offered in the `open_root_actor()` and `.testing._pytest` harness override fixture.