index-v7

Interscript ML model index (index-v7).

Adds since index-v6:

• ara-diac-plane-1.0 — plane-factorized small (219 MB int8, full-set 3.5905 DER)
• ara-diac-plane-large-1.0 — plane-factorized large (866 MB int8, full-set 2.7397 DER, dedicated #2)

New field: kind: plane for plane-factorized artifacts (resolved by the
plane runtime; loader contract = metadata.yaml + plane.onnx +
classes.json with member sha256s).

Runtimes resolve DEFAULT_INDEX_URL against this release asset + sha256
sidecar.