pipelines: replace stderr prints with warnings.warn in library functions

[shaypal5]

Context

PR #30 moved pipeline functions from scripts/build_v5_snapshot.py into leadforge/pipelines/build_v5.py. These functions (subsample() in particular) print warnings directly to sys.stderr.

Now that they live in the package, downstream callers can't easily suppress or redirect this output. Library code should use warnings.warn() or a logger instead.

What to do

1. Replace print(..., file=sys.stderr) calls in leadforge/pipelines/build_v5.py with warnings.warn(..., stacklevel=2)
2. Update tests in tests/scripts/test_build_v5_snapshot.py to use pytest.warns() instead of capsys.readouterr()
3. Optionally keep the print in the CLI wrapper (scripts/build_v5_snapshot.py) for user-facing progress messages

Origin

Copilot review comment on PR #30.