LAB nightly: produce release-readiness-v2 evidence and grade it with pv extract --v2-evidence (Refs #4539) #5193

Parent: #4539 (KREG)

The release-readiness-v2 SHACL shapes that #5187 adds (model-smoke and the related cells) grade only the output of pv extract --v2-evidence <dir> (crates/aprender-contracts-cli/src/cli.rs:686). No workflow, script or Makefile target produces that evidence today, so the shapes grade 0 focus nodes on every run.

Scope:

• Add a producer that writes the v2 evidence directory, then run pv extract --v2-evidence and the SHACL conformance check on it.
• It runs only as a nightly LAB row on an idle machine. It never runs in a PR, merge-queue or release job.
• A red opens or updates this one ticket under one owner and stays silent otherwise. If it is red or unmeasured for 7 days, it stops running until its owner brings it back.
• Promotion to a gate needs an explicit maintainer yes, three green nights on main first, and one existing gate retired.

Falsifier: plant one v2 evidence row that violates a shape (for example a model-smoke cell with no receipt). The nightly row must turn RED on it and GREEN without it.

Owner: la-0.71.1b. No new Python; the producer is Rust or shell.

Agent: aprender-a3 (drafted by la-0.71.1b; follow-up from the #5187 gate-change sign-off minority finding)