v0.6.0 - C ABI

Public C ABI for downstream extensions, enabling C-level participation
in behavior-oriented concurrency across worker sub-interpreters.

New Features

• Decorator composition with @when — decorators stacked below
  @when are now preserved on the generated behavior function and
  compose with the behavior body on the worker. Decorators placed
  above @when raise a SyntaxError at transpile time with
  actionable guidance. async def functions with @when are
  also explicitly rejected.
• Public C ABI (<bocpy/bocpy.h>) — downstream C extensions can
  now link against bocpy to register custom Python types as
  cross-interpreter shareable so :class:Cown can carry instances of
  them across worker interpreters. The header is C-only, version-gated
  via the BOCPY_ABI macro, and bumped on any incompatible change
  to bocpy.h or xidata.h. Wheels remain CPython-version-tagged
  so a runtime ABI mismatch cannot occur.
• bocpy.get_include() / bocpy.get_sources() — Python-level
  helpers that downstream setup.py files use to locate the bocpy
  headers and the small set of C sources that must be compiled into
  the consuming extension.
• templates/c_abi_consumer/ — a ready-to-copy template for
  building a C extension against the bocpy ABI, including a
  setup.py, a probe extension exercising the public surface, and
  a pytest suite (test_public_c_abi.py) that validates the ABI
  end-to-end.
• C source reorganisation — the per-subsystem translation units
  introduced in 0.5.0 have been renamed with a boc_ prefix
  (boc_compat.[ch], boc_sched.[ch], boc_tags.[ch],
  boc_terminator.[ch], boc_noticeboard.[ch], boc_cown.h)
  to give the public ABI a stable, namespaced identity. xidata.h
  has moved under include/bocpy/ alongside bocpy.h.

Documentation

• New :doc:c_abi, :doc:messaging, and :doc:noticeboard pages
  in the Sphinx site; the API reference has been expanded to cover
  the public ABI surface.

Breaking Changes

• noticeboard_version removed — the global monotonic version
  counter introduced in 0.4.0 has been removed. It exposed an
  implementation detail of the snapshot cache that did not survive
  the C ABI review and had no use case that was not better served
  by notice_sync plus an explicit noticeboard() read.