Publish to PyPI on a v* tag

[remislp]

dcio not being on PyPI has become a running cost: it appears in EKDIST's
install_requires where pip cannot resolve it, in two CI workflows that
install it from git before anything else, and in HJCFIT's ImportError text.
Publishing removes all three.

The name dcio is available on PyPI.

dcio had no licence at all

No LICENSE file, no declaration in pyproject.toml, nothing in the README.
That means all rights reserved by default — nobody could legally use what
was about to be uploaded.

Added MIT. The reasoning matters more than the choice: dcio sits underneath
SCALCS (GPL-2.0-only) and HJCFIT (GPL-3.0), and those two are mutually
incompatible. Any copyleft licence on the foundation would conflict with one
of its own consumers, and GPL would additionally force EKDIST (MIT today) to
change. A permissive licence at the bottom of the stack constrains nobody.

Packaging metadata

readme, license, license-files, classifiers, keywords and project URLs.
Without a readme the PyPI page would have rendered as a bare title.

Publishing

No API token in the repository. PyPI verifies the workflow's OIDC identity
instead (Trusted Publishing), so there is no secret to leak, rotate, or hand
to anyone.

A version cannot be reused once uploaded, so publish runs only on a v*
tag. Every push still builds and checks, so a tag is only ever cut from
something already known to build.

Three build gates:

• twine check --strict — catches metadata that renders badly or fails upload;
• exactly one py3-none-any wheel — dcio is pure Python and should stay
  that way; if it ever stops being true, the wheel filename changes and this
  notices;
• the wheel builds from the unpacked sdist alone — the only way to catch a
  file missing from the source distribution.

Verified locally

Both artifacts build, twine check --strict passes on both, the wheel carries
all four subpackages plus the licence, and the Qt viewer is correctly excluded.

What you need to do

I have not touched any credential and will not. Before the tag can publish,
register a pending publisher at https://pypi.org/manage/account/publishing/:

field	value
PyPI project name	dcio
Owner	DCPROGS
Repository name	dcio
Workflow name	release.yml
Environment name	pypi

Once that exists and this PR is merged, tell me and I will tag v0.1.0.

🤖 Generated with Claude Code