Drop variant B, adopt Black-belt Exam defender, add one-sidedness metrics

- Remove variant B (Brown cap, +1 for higher beating lower) from rules,
  engine, bots and simulator; a higher belt beating a lower one gains 0.
- Final Exam: the defender fights as a Black belt (RULES.md v0.7). Ties to
  the defender and best-of-3 stay as simulator switches (`--exam`).
- Report stripe income per source as gains and losses.
- Add ten one-sidedness measures sampled at turn ends (lead gap, winner
  margin and lead share, halfway leader, last lead change, contention,
  rank swaps, leader Kumite losses, Exam challengers, winner pace).
- Plan: decisions D28-D33 and the detailed Phase 6.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/test_cli.py

@@ -62,19 +62,21 @@ def test_sim_prints_a_summary_per_scenario_and_writes_games(
     tmp_path: Path, capsys: pytest.CaptureFixture[str]
 ) -> None:
     games = tmp_path / "games.csv"
-    argv = ["sim", "--games", "2", "--variant", "A", "B", "--players", "3", "--jobs", "1"]
+    argv = ["sim", "--games", "2", "--exam", "c", "none", "--players", "3", "--jobs", "1"]
 
     assert main([*argv, "--csv", str(games)]) == EXIT_OK
 
     output = capsys.readouterr().out
-    assert "== A p3 kiai+2 start0 balanced,farmer,aggressive  (2 games" in output
-    assert "== B p3 kiai+2 start0" in output
+    assert "== exam:none p3 kiai+2 start0 balanced,farmer,aggressive  (2 games" in output
+    assert "== exam:c p3 kiai+2 start0" in output
     rows = games.read_text(encoding="utf-8").splitlines()
     assert rows[0].startswith("scenario,seed,lineup,winner_seat")
     assert len(rows) == 1 + 4
 
 
-@pytest.mark.parametrize("bad", [["--lineup", "balanced,cheater"], ["--games", "0"]])
+@pytest.mark.parametrize(
+    "bad", [["--lineup", "balanced,cheater"], ["--games", "0"], ["--exam", "b"]]
+)
 def test_sim_rejects_bad_arguments(bad: list[str], capsys: pytest.CaptureFixture[str]) -> None:
     with pytest.raises(SystemExit) as exit_info:
         main(["sim", *bad])