ONNX-style producer fields, rename producers to packages (wire format v3)

[Gnpd]

Summary

• Bumps the wire format to v3: metadata.producer_name/producer_version now follow ONNX semantics and name the tool that wrote the file ("openmodels" + its release version, or a third-party writer's own name/version) instead of conflating the outermost model's package with scikit-learn's version.
• scikit-learn's version moves to a new metadata.domain_version field, which the deserialize-time version check now reads (falling back to producer_version only for v1/v2 files, since that's the only shape where it ever held the sklearn version).
• metadata.producers is renamed metadata.packages and now always includes sklearn, even when no estimator in the tree is scikit-learn's own class. v2 files' producers key is still read for backward compatibility.
• Deserialize now also warns (never fails) on a version mismatch for any non-sklearn package listed in packages.
  • metadata.dependency_versions gains python (platform.python_version()).
  • Drops metadata.openmodels_version (v3 only) - producer_version with producer_name: "openmodels" records the same release, and the old field was never read on deserialize.
  • docs/format.md updated: field table, worked examples, a new "Files written by other tools" section, and format history entry for v3.
  • Releases as 0.2.2 (CHANGELOG + pyproject.toml).

Why

The old producer_version/producer_name pair mixed two unrelated things - which package wrote the model class, and what scikit-learn version its attribute layout matches - into one field. That broke down for a third-party root estimator (mismatched values) and gave a non-Python writer (e.g. an R exporter) nothing honest to put in producer_name. Splitting "who wrote this file" (producer_*) from "what environment it targets" (domain_version, packages) follows ONNX's convention and makes both openmodels-written and externally-written files self-describing.