substrate-step-test: plant a template stored with the previous wire format before the step #568

Problem

The step check's "agents from before the step" are templates the current kagent wrote, so a persisted-format gap (here the singular trust-bundle name that Substrates before 1.6.0 stored, which the 1.8.x atelet read as an empty names list and panicked on) never reaches the lab. The format gap passed every lab proof and showed up only on an installation holding old rows.

Proposed solution

A fixture plants an old-format row (the old wire field) before the step, so substrate-step-test boots a golden from it after the step, for every persisted format a Substrate or kagent step changes.

Acceptance criteria

• The step check includes a template stored with the previous wire format and passes only if its golden boots after the step.

This issue was written by an agent.