chore: fresh bare pytest tests, de-classed suites and new fixture values - #1866

Summary

Follow up to #1865 for #1835, addressing the independent adversarial audit. Three parts:

1. Freshly authored bare pytest replacements for the deleted 2009 descended coverage: parametrized tests for deep label chains, the oversized label rejection, duplicate questions, empty message roundtrips, header flag bits, the multicast zero id, section counts, and question equality.
2. The remaining class based tests in test_init, test_protocol and test_core are converted to bare pytest functions, which retires the 2009 class names Framework, Names, PacketGeneration and PacketForm; unittest skip decorators became pytest.mark.skipif and one call form pytest.raises became the context manager form. The conftest known blocking nodeid was updated for the de-classing.
3. The 2009 descended fixture values are replaced across the whole suite: ash-2.local becomes spare-rig.local, the 10.0.1.x address family becomes 10.7.4.x, and testname.local becomes candidate.local.

An AST structure comparison of the fresh tests against the 2009 test file shows only the generic build call parse shape every tiny roundtrip test shares; the class names, method sequence, fixtures and assertion style that identified the port are gone.

Test plan

• full suite passes (513, fresh parametrized cases replace the deleted methods)
• fixture sweep confirms no 2009 fixture strings remain