Such that we can run (opting-in) tests on both TCP and UDS backends and
ensure the `reg_addr` fixture and various timeouts are adjusted
accordingly.
Impl deats,
- add a new `tpc_proto` CLI option and fixture to allow choosing which
"transport protocol" will be used in the test suites (either globally
or contextually).
- rm `_reg_addr` instead opting for a `_rando_port` which will only be
used for `reg_addr`s which are net-tpt-protos.
- rejig `reg_addr` fixture to set a ideally session-unique `testrun_reg_addr`
based on the `tpt_proto` setting making appropriate calls to `._addr`
APIs as needed.
- refine `daemon` fixture a bit with typing, `tpt_proto` timings, and
stderr capture.
- in `test_discovery` do a ton of type-annots, add `debug_mode` fixture
opt ins, augment `spawn_and_check_registry()` with `psutil.Process`
passing for introspection (when things go wrong..).
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