…_ self-tests

compute_cells() expands a test's stacked @requirement marks into the
(transport, spec_version) pytest.param list with intersection semantics:
a cell is dropped if any requirement's [added_in, removed_in) window or
arm_exclusions excludes it, and marked xfail-strict if any requirement's
known_failures matches it. Requirement.transports is intentionally not
consulted (descriptive metadata only). cell_id() keeps node-id suffixes
as bare transport names while SPEC_VERSIONS has a single entry.

test_coverage.py gains pytest.raises self-tests for every new
__post_init__ validation branch on ArmExclusion / KnownFailure /
Requirement.

12 passed; pyright/ruff clean; conftest not yet wired.

pytest_generate_tests now reads each connect-taking test's stacked
@requirement marks, looks them up in REQUIREMENTS, and parametrizes the
connect fixture with compute_cells() output. The fixture body unpacks
the (transport, spec_version) tuple and returns the bare factory.

411 connect-parametrized cells with byte-identical [transport] node ids;
no behaviour change while SPEC_VERSIONS has a single entry.

test_coverage.py gains 10 unit tests for compute_cells/cell_id covering
intersection semantics, wildcard arm-exclusion / known-failure matching,
TRANSPORT_SPEC_VERSIONS gating, and the transports-field-is-metadata-only
guarantee. _requirements.py is at 100% line+branch coverage from
test_coverage.py alone.

test_coverage.py gains five manifest-level invariants (vacuously green
until entries start using the new fields):
- SPEC_VERSIONS subset of KNOWN_PROTOCOL_VERSIONS and includes LATEST
- CONNECTABLE_TRANSPORTS == conftest._FACTORIES keys
- supersedes / superseded_by links are bidirectional and versioned
- every arm_exclusion / known_failure targets a reachable cell

README.md: the requirements-manifest field list documents the new
fields, and the spec-revision section is rewritten as 'Spec versions
and the era axis' covering compute_cells() intersection semantics, the
transports-is-metadata-only rule, and the checklist for landing a new
revision.

Full gate: 2299 passed, 100.00% coverage, pyright/ruff clean,
411 connect cells with unchanged ids.