fix: require pandas>=2.3.3 on Python 3.14

[hiro-o918]

Background

Investigating why only the tests (ubuntu-latest, py314) job failed on #529 revealed the root cause is pandas, not pillow.

• The failure is a segmentation fault inside pandas' datetime C extension at test/test_pandas_type_config.py::test_datetime_success, deterministically reproducible on re-run: https://github.com/m3dev/gokart/actions/runs/29820432602
• pandas 2.2.3 ships no cp314 wheels, so on Python 3.14 it is always built from sdist — an unsupported, untested configuration (the first pandas release with cp314 wheels is 2.3.3)
• A full diff of all 82 installed packages between the last green master run and the failing run shows the only change is pillow 12.2.0 → 12.3.0. Pillow 12.3.0's C-extension optimizations changed the process memory layout, which exposed latent undefined behavior in the sdist-built pandas — pillow is the trigger, pandas is the actual defect.

Changes

• pyproject.toml: split the pandas requirement with environment markers — unchanged (unconstrained) for python_version < '3.14', pandas>=2.3.3 for python_version >= '3.14'
• uv.lock: regenerated. pandas is forked to 2.3.3 (cp314 wheels) for Python >= 3.14 while staying at 2.2.3 for <= 3.13. No other package versions changed (only mechanical resolution-marker splits).

QA

• uv sync --locked -p 3.14 --group test installs pandas 2.3.3 from a prebuilt cp314 wheel (Tag: cp314-cp314-macosx_11_0_arm64)
• test/test_pandas_type_config.py (previously segfaulting) passes all 6 tests on Python 3.14
• uv sync --locked -p 3.12 still resolves pandas 2.2.3 (resolution for <= 3.13 untouched); the same tests pass
• uv run ruff check . / uv run ruff format --check . / uv run mypy gokart test all pass
• Marker boundary conditions evaluated with packaging.markers.Marker.evaluate() (3.13.9 → old branch; 3.14.0rc1 / 3.14.0 / 3.15.0 → new branch; mutually exclusive and exhaustive)

[hirosassa]

LGTM!