Score canonical legacy abstentions as explicit misses

When a trusted legacy baseline lacks a route or node, preserve its canonical abstention as an explicit grounded prediction that scores zero; never drop the row. Require an exact sentinel shape, matching node, empty arguments, non-executable status, and signed provenance. Unknown routes or node mismatches must still fail closed. Otherwise baseline coverage can shrink silently, making regression gates either crash or become artificially easy. This compatibility transform should itself be hash-bound in the selection receipt.

Applies when: comparing a new structured-action router against an older baseline with incomplete output-space coverage. Keywords: baseline abstention unsupported route regression gate scorer provenance