fix: declare a PyYAML floor that installs on Python 3.14

[lesnik512]

Closes #126.

Reproduced, then measured rather than guessed

The issue's repro fails exactly as described — PyYAML>=6 resolves to 6.0, which has no cp314 wheel, so uv falls through to the sdist:

AttributeError: 'build_ext' object has no attribute 'cython_sources'
hint: `pyyaml` (v6.0) was included because `compose2pod[yaml]` (v0) depends on `pyyaml`

The issue says to find the fixed version by installing candidates rather than guessing, and that turned out to matter — the obvious answer is wrong. On 3.14:

PyYAML	installs	installs with --only-binary
6.0	no	no
6.0.1	yes	no
6.0.2	yes	no
6.0.3	yes	yes

6.0.1 and 6.0.2 install on this machine, so a floor of >=6.0.1 would look green here and in CI. They install by compiling the sdist, and only because a toolchain happens to be present. The README's pitch is "installs and runs in minimal Python images", where it is not. 6.0.3 is the first release with a cp314 wheel (confirmed against the PyPI index: cp310, cp311, cp312, cp313, cp314, cp38, cp39), and that is the floor a user of such an image can actually install.

The fix

Marked per interpreter, as the issue proposes, rather than raised for everyone — 6.0 works fine below 3.14 and an older interpreter has no reason to lose the lower floor:

yaml = [
    "PyYAML>=6; python_version < '3.14'",
    "PyYAML>=6.0.3; python_version >= '3.14'",
]

Verified both ends, binary-only, with --resolution lowest-direct:

3.10 -> PyYAML 6.0     import ok
3.14 -> PyYAML 6.0.3   import ok

The gate, because "nothing reports it" is the actual bug

The floor was unsatisfiable for as long as 3.14 has been supported and no gate noticed, because every job here resolves highest. A one-instance fix leaves that intact, so this adds a floors job: uv pip install --resolution lowest-direct --only-binary PyYAML ".[yaml]" on the oldest and newest supported interpreters, then an import.

--only-binary is the load-bearing part. Without it this job would have gone green on a >=6.0.1 floor that no minimal image can install — the table above is what that flag buys.

3.14t is deliberately out of the matrix. No PyYAML release ships a cp314t wheel, so a binary-only floor check there would fail on upstream's gap rather than on this repo's floor. Free-threaded users build PyYAML from source today regardless of what this project declares; that is unchanged by this PR and not something it can fix.

This is beyond the issue's literal ask, which was the marker alone. Flagging it explicitly: if you would rather ship the two-line fix and track the gate separately, say so and I will split it.

Verification

• just lint, just test-ci: green, 100% line coverage held.
• The four install outcomes above were run locally on real 3.10 and 3.14 interpreters, not inferred from wheel filenames — the filename check came after, to explain them.