fix: stop rendering "-0.0 pp" for tiny negative share changes in compare

[propcgamer20-png]

Problem

faircode/report.py renders a misleading -0.0 pp whenever a share_delta is a tiny negative float that rounds to 0.0 at one decimal place. Both surfaces are affected:

• compare_to_terminal - the {g['share_delta'] * 100:+5.1f} column
• compare_to_html's signed() helper - prefix = "+" if val > 0 else "" then f"{val:.1f}"

Repro (before)

$ python3 -m faircode compare a.csv b.csv
  female              50.0% ->  50.0%  ( -0.0 pp)
  male                50.0% ->  50.0%  ( +0.0 pp)

-0.0 pp next to 50.0% -> 50.0% reads as a rendering bug.

Fix

New module-level helper:

def _strip_neg_zero(val: float, dp: int = 1) -> float:
    return 0.0 if round(val, dp) == 0 else val

Applied in compare_to_terminal (share_delta column) and in signed() (so the HTML share-delta and score-delta paths are both covered). Non-zero values are untouched, so no other output changes.

After

  female              50.0% ->  50.0%  ( +0.0 pp)
  male                50.0% ->  50.0%  ( +0.0 pp)

Test

test_compare_does_not_render_negative_zero_share_delta - asserts -0.0 pp appears in neither compare_to_terminal nor compare_to_html for a dimension whose group share moves by -0.0001.

pytest tests/test_report.py tests/test_compare.py -> 39 passed. ruff clean.

Closes #515

🤖 Generated with Claude Code

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