release: 0.5.0

[andremsouza]

Version bump and changelog for 0.5.0. One change, no results move.

What it is

Plain-string options now emit DeprecationWarning:

Passing mode='batch' as a plain string is deprecated and will stop working in 1.0.0.
Use TrainingMode.BATCH instead.

That is the whole release. Deliberately single-purpose, so the one behaviour change is easy to see
and easy to revert if it turns out to be noisy in practice.

Why it has to exist

1.0.0 was blocked on it. The stated policy is that anything removed in a major release warns for
at least one minor release first. 0.4.0 deprecated strings in writing only, because mode="batch"
was what every documentation page showed at the time. Without this release, 1.0.0 removing strings
would breach the policy the project set for itself.

Nothing else moves

No numerical change, no API removal, no dependency change. Anything that worked in 0.4.0 works here
and produces identical output. The only observable difference is the warning, which names the exact
replacement so migrating is a substitution you read off the message.

To silence it while migrating:

import warnings
warnings.filterwarnings("ignore", category=DeprecationWarning, module="python_som")

Verified from the built wheel

installed set:   numpy==2.5.1, python-som==0.5.0
enums:           train under -W error::DeprecationWarning, no warning
plain strings:   warn, naming the replacement

Plus 384 tests at 100% coverage, ruff / ruff format / mypy --strict clean, mkdocs build --strict exits 0, twine check passes both artifacts.

After merge

Tag v0.5.0, then the workflow builds and waits on the pypi environment's required reviewer. The
upload is irreversible — I will ask before approving it.