No dependency floor in pyproject.toml is tested

pyproject.toml declares eight >= floors. None of them is tested — every CI
leg installs the newest of everything, so each floor is a claim nobody has
checked.

requires-python = ">=3.11" is the exception, and is well covered: test.yml
runs a 3.11 leg on ubuntu and windows, [tool.mypy] python_version = "3.11"
type-checks against it, and CONTRIBUTING tells developers to develop on it. The
exact pins (mypy, ruff, build, twine) enforce themselves.

lxml>=5.0 is the one that matters to users: it is the only runtime dependency,
and the floor is a public compatibility promise. Nothing has ever installed
lxml 5.0 against this test suite.

Proposal

One CI leg on the Python floor that installs the declared minimums and runs the
suite:

  floors:
    runs-on: ubuntu-latest
    # python-version: "3.11"
    - run: python -m pip install -e .[dev] -c floors.txt
    - run: python -m pytest

floors.txt lists each declaration as == (lxml==5.0, pytest==8.0, …).
pip has no --resolution lowest — that is uv — so a constraints file is the
mechanism. The build-backend floor needs PIP_CONSTRAINT instead, since it is
resolved into build's isolated environment rather than the install
environment.

Expect this to fail at first and to move some floors up. That is the point: a
floor that fails is a floor that was wrong.

Related: the license-metadata assertion

Hatchling before 1.27 builds this project's wheel without complaint and emits
license = "MIT" as a legacy free-text field, dropping License-Expression and
License-File; twine check passes that wheel (see #24). A floor alone cannot
catch it, so build.yml and release.yml want a step asserting those two fields
are present in the built wheel — about nine lines of YAML each, verified to pass
a 1.32.0 wheel and fail a 1.26.3 one. It belongs with this work rather than
ahead of it: it would otherwise be the only enforced package floor in the file.

Start with pyproject.toml and the existing test.yml, docs.yml, build.yml, and release.yml workflows, then review how floors.txt constraints interact with normal and isolated installs. Add a Python 3.11 CI leg that exercises the declared minimums, and add the described wheel metadata assertions where appropriate; done means the suite passes at the floors and the assertions distinguish the supported Hatchling versions.