A feature-gated test file reports green when the feature is off, so it can protect nothing

[macanderson]

Problem

Both cross-language vector suites in contextgraph-types are gated at file
scope:

• contextgraph-types/tests/attestation_vectors.rs — #![cfg(feature = "attestation")]
• contextgraph-types/tests/record_vectors.rs — #![cfg(feature = "record-attestation")]

When the feature is off, the file compiles to zero tests and cargo prints
test result: ok. 0 passed. That is a green line for a suite that ran nothing.
Both files exist to pin wire-format bytes, and both are the loudest thing that
should fail if a preimage rule changes — a shape that reports success when it
was not built is the opposite of what they are for.

Observed while verifying PR #114's witness. On a tree without the
record-attestation feature, cargo test -p contextgraph-types --all-features --test record_vectors printed test result: ok. 0 passed rather than failing.

Why the existing mitigation is not enough

PR #114 added a features CI job that runs each feature combination by name, so
today the vectors are exercised in CI. That defends the current file list; it
does not defend the next one. A new gated test file added without a matching CI
line is silently inert, and nothing says so.

Files

• contextgraph-types/tests/attestation_vectors.rs
• contextgraph-types/tests/record_vectors.rs
• contextgraph-types/Cargo.toml (the feature list)
• .github/workflows/ci.yml (the features job)

Reproduce

git stash push -- contextgraph-types/Cargo.toml   # drop the feature definitions
cargo test -p contextgraph-types --all-features --test record_vectors
# test result: ok. 0 passed  <- green, ran nothing

Options

1. Keep the file ungated and gate only the test bodies, leaving one always-on
   test that asserts the feature list is as expected. A missing feature then
   fails loudly.
2. Add a guard that reads contextgraph-types/Cargo.toml's [features] and
   fails if the features CI job does not name every one of them, so the job
   cannot fall behind the crate.
3. Both. (2) is the durable half — it makes the CI job's coverage checkable
   rather than a thing someone remembers.

Done

A gated vector file that is not built by CI causes a failure somewhere, rather
than a green line reporting zero tests. Whichever option is taken carries a test
of its own that fails when the coverage lapses.

[macanderson]

One branch of this narrows in PR #154, and it is worth recording which: contextgraph-conformance now depends on contextgraph-types with features = ["attestation"], so cargo's feature unification turns the feature on for the ordinary workspace build. tests/attestation_vectors.rs therefore compiles and runs its 5 tests in the plain test job, not only in the features job.

That is a mitigation, not a fix, and it is exactly the shape this issue warns about — the file is protected today by a coincidence of the dependency graph, and a change that dropped the feature from that dependency would silently return it to 0 passed. record_vectors.rs is unaffected and still relies on the features job alone. The structural fix this issue asks for — a gated file that fails rather than reporting green when its feature is off — is still needed.