Release 0.3.0

Added

• _deep_resolve_refs() — recursive $ref resolution for nested OpenAPI schemas,
  handling allOf/anyOf/oneOf, items, and properties. Depth-limited to 16
  levels to prevent infinite recursion on circular references.
• Enhancer protocol — pluggable interface for metadata enhancement, allowing
  custom enhancers beyond the built-in AIEnhancer.
• HTTPProxyRegistryWriter — registers scanned modules as HTTP proxy classes
  that forward requests to a running web API. Supports path parameter substitution,
  pluggable auth headers, and 2xx success range (with 204 returning {}).
  Requires optional httpx dependency (pip install apcore-toolkit[http-proxy]).
• get_writer("http-proxy", base_url=...) — factory support for the new
  HTTP proxy writer with **kwargs forwarding.
• Expanded __init__.py public API: exports Enhancer, HTTPProxyRegistryWriter,
  WriteError, Verifier, VerifyResult, verifier classes, serializer functions,
  resolve_ref, resolve_schema, extract_input_schema, extract_output_schema,
  and run_verifier_chain.

Fixed

• extract_output_schema() — now recursively resolves all nested $ref pointers
  (previously only handled the shallow case of array items with $ref).
• extract_input_schema() — now recursively resolves $ref inside individual
  properties after assembly.
• get_writer() return type annotation now includes HTTPProxyRegistryWriter.

Tests

• 272 tests (up from 260), all passing
• Added TestDeepResolveRefs (8 tests): top-level ref, nested properties,
  allOf/anyOf, array items, deeply nested refs, circular ref depth limit,
  immutability guarantee
• Added nested $ref tests for extract_input_schema and extract_output_schema
• Added test_http_proxy for get_writer("http-proxy") factory

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