The lane shipped dead. `classify_changes.py` emitted `rust`, the composite
action re-exported it, and ci.yaml's `rust-tests` job gated on
`needs.detect.outputs.rust` — but the `detect` job never declared that
output, so the expression was the empty string and the job reported
"skipping" on the very PR that added it. GitHub does not error on a
reference to an output a job never declared, so nothing went red.
Adds the missing line plus the invariant that catches the whole class:
every `needs.detect.outputs.X` referenced by a job's `if` must be
declared by `detect`. Verified it fails with the line removed.
The related check — every lane reaching the composite action — is
separate on purpose: nix.yml and docker.yml own their triggers and
re-export different subsets, so `docker` and `nix` are legitimately not
ci.yaml detect outputs.
dd1e5b723d