Ground truth validation: Phase 2 roadmap (coverage, gradients, numcrux) #984

Parent tracking issue for the follow-up work identified while wiring
skainet-test-groundtruth into real CI (see the PR that adds
.github/workflows/ground-truth.yml, closes the metadata-parsing gap, and adds
-PrequireGroundTruth). That change made the existing pipeline actually run and
actually fail loud when broken — it did not expand what the pipeline covers. This issue
tracks what's next.

Related: #447 ("Wire IREE execution into CI and gate on PyTorch numerical checks") is
the same idea for the compiled (IREE) path — this issue's Phase 1 work is the eager-path
analog, already landed. #321 (closed) has historical context on the 3-repo layout.

Sub-issues

• Expand Python-side op coverage
• Decide on gradient validation
• Numcrux transfer/donation (governance)
• (stretch) Single-source DSL generating both Kotlin and Python fixtures

Filed as separate linked issues so each can be picked up independently.