ArtifactOf pydantic integration + registry.extra reorg (0.6.0)

[adnanhd]

Summary

Ships v0.6.0: a pydantic-native ArtifactOf type, a core/extra split for the
observability batteries, and a coverage pass.

Changes

• ArtifactOf[Registry] (registry.integrations.pydantic): a field or
  @validate_call argument that accepts a live artifact of the registry's
  element type or a config to build, serializes mode-aware (python -> artifact,
  json -> the {type, data, meta} config), and accepts a union of registrars
  (ArtifactOf[A, B]). Its validator threads already-validated siblings
  (info.data) into the build scope, so a $sibling.attr() ref (e.g. an
  optimizer over $model.parameters()) resolves against the live sibling.
• Core is bus-only: concrete meters (CPU/Memory/IO/Network/Heap/Recursion/
  Lifetime) and reporters (Journal/HTTPDashboard/OpenTelemetry) move to
  registry.extra; the core keeps the attach/detach/emit buses.
• torch_compat -> examples: the torch demo (state-dict hashing, tensor-shape
  markers, the profiler meter) leaves the shipped package for examples/.
• Coverage: raised to 85% with tests for the CLI, _version, engines,
  utils, and the validator mixin.

Breaking

• Concrete meters/reporters import from registry.extra.meters /
  registry.extra.reporters (were top-level).
• registry.experimental is gone; torch_compat lives in examples/.

Verification

pytest: full suite green except one pre-existing torch-profiler test
(unrelated -- a renamed torch attribute on this Python build).

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 81.87919% with 54 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
registry/extra/reporters.py	62.37%	38 Missing ⚠️
registry/extra/meters.py	89.62%	14 Missing ⚠️
registry/integrations/pydantic.py	96.61%	2 Missing ⚠️

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