ci: install the declared floors wheel-only without letting the resolver climb

[lesnik512]

The floors job now installs each declared floor exactly, so a floor with no wheel fails the job instead of passing on a higher version. It uses two uv commands and no script.

The gap

--only-binary PyYAML doesn't make a floor with no wheel fail. It makes that version ineligible, and --resolution lowest-direct then climbs to the lowest version that has a wheel, then passes. This is how PyYAML>=6 resolved 6.0.1 and 6.0.2 on 3.12 and 3.13 while the job stayed green (#135). uv has no option that turns the climb into an error. Its maintainers treat skipping versions without wheels as intended (astral-sh/uv#16186).

The fix

- run: uv pip compile pyproject.toml --all-extras --no-deps --resolution lowest-direct -o floors.txt
- run: uv pip install -r pyproject.toml --all-extras --constraints floors.txt --no-build

1. Pin the floors. The compile resolves each direct dependency's floor for this interpreter with builds allowed, so a version with no wheel isn't skipped. --no-deps limits the output to direct dependencies.
2. Install them wheel-only. A floor with no wheel then fails with uv's own explanation.

-r pyproject.toml installs the dependencies but not the project. So --no-build needs no carve-out, and the smoke import reads compose2pod from the checkout.

This follows what larger projects do: sunpy, jwst, napari, scikit-image and scikit-learn all test exact minimum pins rather than a lowest resolution. It needs no generator script, so the rollout to other repos is a change to _checks.yml only, which also works for repos that currently use uv sync --no-install-project.

Verified

• CI on this PR: all five floors legs pass.
• Planted bug on CI: the throwaway draft DO NOT MERGE: prove the floors job fails on a wheel-less floor #140 set the 3.12 floor back to PyYAML>=6. Only floors (3.12) failed, at the wheel-only install step; every other check passed. DO NOT MERGE: prove the floors job fails on a wheel-less floor #140 is closed.
• Locally, with Linux wheels: 3.10 to 3.14 install PyYAML 6.0, 6.0, 6.0.1, 6.0.2 and 6.0.3, each interpreter's declared floor. The planted case fails with pyyaml==6.0 has no usable wheels ... unsatisfiable.

Known limits

• A floor excluded by requires-python. The compile would still move up past a declared floor whose own requires-python excludes this interpreter.
• A dependency with no floor. The compile warns that it is unpinned and pins an ancient version, which then fails the wheel-only install. The failure is loud, but the message doesn't name the cause. Lint for published dependencies declared with no lower bound .github#108 tracks a proper lint for it.