ci: record why the floors gate omits 3.14t

[lesnik512]

The lowest job omits 3.14t, which the pytest job runs. Nothing said why.
§7 requires the gate on
"every matrix entry", permits dropping "a matrix entry no upstream wheel covers", and closes with
"Drift that nobody recorded is not an exemption." The omission is justified; it was just never
written down.

Verified

No aiokafka release ships a free-threaded wheel. 0.13.0 and 0.14.0 are the only releases with
any cp314 wheel, and both carry cp314-cp314 only, no cp314t and no py3-none-any:

aiokafka-0.14.0-cp314-cp314-manylinux2014_x86_64...whl
aiokafka-0.14.0-cp314-cp314-macosx_11_0_arm64.whl
...
sdist: aiokafka-0.14.0.tar.gz

So the gate cannot pass there, by construction:

$ uv python pin 3.14t
$ uv sync --all-extras --no-install-project --resolution lowest-direct --no-build
error: Distribution `aiokafka==0.14.0 @ registry+https://pypi.org/simple` can't be installed
because it is marked as `--no-build` but has no binary distribution

pytest reaches 3.14t only because it has no --no-build: it builds the sdist with
AIOKAFKA_NO_EXTENSIONS=1, giving a pure-Python install. The floors gate forbids exactly that, and
should, so the entry measures upstream's gap rather than this repo's floor.

What this does not change

Comment only. No matrix, flag or dependency change, so there is nothing new to run locally; CI is
unchanged in behaviour.

The matching exemptions-table row belongs in modern-python/.github alongside §7's own table, not
here. It is batched into a standard PR covering the rest of this sweep, per §7's rule that a core
change "lands as one pull request that updates this page and the exemptions table together."

Lift when an aiokafka release ships a cp314t wheel.