containment-broker: a malformed attestation escapes validatePrelaunchAttestation as a bare TypeError instead of an enumerated refusal

[thegeorgepu]

Found by adversarial review of #30 and reproduced locally — verified, not a report.

In the capability loop at vinci/containment-broker/src/admission.mjs:119-121 the guard is written
sortedUniqueStrings(capabilities ?? []), but the very next line uses sha256(capabilities) with no
?? []. When an attestation omits capabilities (or one of the five sets), capabilities is
undefined, sortedUniqueStrings([]) returns true, so the else if runs and
canonicalBytes(undefined) throws.

$ node -e 'validatePrelaunchAttestation({ attestation: { no_new_privs: true }, policy: {} })'
threw: TypeError | code: (none) | reasons: (none)
message: unsupported canonical value: undefined
CONFIRMED — escapes as a bare TypeError, not AdmissionRefusedError

Why it matters. validatePrelaunchAttestation is a refusal path: it is supposed to fail with
AdmissionRefusedError carrying code and an enumerated reasons list. A malformed attestation
instead escapes as a bare TypeError with neither. Any caller branching on e.code === "CONTAINMENT_ADMISSION_REFUSED"
mishandles it, and the enumerated refusal reasons — the thing that makes a refusal auditable — are lost.
A missing field should be a reason, not a crash.

Every sibling check in the same function (limits, supplementary_groups, inherited_fds,
ingress, controller_objects) is guarded. This one is the outlier.

Suggested fix. sha256(capabilities ?? []), and a test that a wholly empty attestation object
produces AdmissionRefusedError with reasons rather than a TypeError.

Not a regression from #30; pre-existing in the v3 boundary.