Every ExUnit test is dead in CI: the only mix test sits behind a needs: edge to a failing job #836

The finding

Every ExUnit test in this repository is dead in CI. Not failing — never
executed, while the workflow, the job and the check-run all still appear.

There is exactly one mix test invocation in the whole tree:

.github/workflows/tests.yml:347   run: mix test --include integration

It lives in the integration-tests job, which declares needs: [build-images].
Build Test Images fails. A needs: edge to a failing job does not fail the
dependent job — it makes it skipped.

Measured on the run for 3bf9674a:

Build Test Images    failure
Integration Tests    skipped      <- the only home of mix test

Why this matters more than a red suite

A red suite is visible. A needs:-skipped job looks like it participated: it is
listed, it has a name, it reports a conclusion. And a skipped conclusion
satisfies a required status check, so this can sit behind a green ruleset
indefinitely.

Concretely, it already has: test/unified-api-adapter-contract_test.exs is the
ABI wire-contract guard that #120 repointed from the dead src/abi/Types.idr
onto the live src/Hypatia/ABI/Types.idr, closing instance 21 of the
guard-reads-the-wrong-thing trap. That fix is correct in the tree and vacuous
in CI, and nothing anywhere said so.

Note also that mix test excludes 242 :verisim_data tests unconditionally
(test/test_helper.exs), so even once it runs, the denominator is ~83%.

Acceptance criteria

1. Integration Tests reaches conclusion success on a run against main — not
   skipped, not cancelled. Either fix build-images, or move mix test out
   from behind it into a job with no dependency on image building. The Elixir
   unit suite does not need ArangoDB; only the :integration-tagged subset does.
2. The fix is proven by a mutant: break one assertion in
   test/unified-api-adapter-contract_test.exs (e.g. change one expected
   connector name), push, and confirm the check goes red. Revert and confirm
   green. A suite that cannot be made to fail is not evidence it runs.
3. The run prints its denominator — the N tests, M failures line must appear
   in the log with N > 0. A zero-test run exits 0 and reads as success.
4. A workflow-level assertion that no job carrying a test runner can resolve
   skipped silently: either drop the needs: edge, or add a final job with
   if: always() that fails when needs.integration-tests.result == 'skipped'.
5. State plainly in the job or the README whether the 242 :verisim_data tests
   are in scope. If they stay excluded, the exclusion is named, not implicit.

Probe for re-checking this class of fault anywhere

git grep -n 'mix test\|cargo test\|zig build test\|bun test' -- .github/workflows
gh run view <run-id> --repo O/R --json jobs --jq '.jobs[]|"\(.conclusion)\t\(.name)"' | sort

Ask "did it execute", never "does it exist".

Found while preparing #835.