Initial prompt to vibe subint support Bo

ai/prompt-io/prompts/subints_spawner.md

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+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 origing the `.spawn.*` subpkg by
+  breaking up various backends currently in `spawn._spawn` in to
+  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
+    with subprocs, but explicit public escape hatches to enable rigorously
+    managed shm channels for high performance apps.
+
+- all tests should be (able to be) paramatrized to use the new
+  `subints` backend and enabled by flag in the harness using the
+  existing `pytest --spawn-backend <spawn-backend>` support offered in
+  the `open_root_actor()` and `.testing._pytest` harness override
+  fixture.