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/scripted.py

@@ -9,7 +9,7 @@ from dojo.board import Board, Pos, build_board
 from dojo.engine import FightView, Game, GameView
 from dojo.grid import parse_grid
 from dojo.movement import Path as Walk
-from dojo.rules import VARIANT_A, Card, RulesConfig
+from dojo.rules import DEFAULT_RULES, Card, RulesConfig
 
 FIXTURE = Path(__file__).parent / "fixtures" / "quadrant_valid.txt"
 NO_CARDS: tuple[tuple[Card, int], ...] = ()
@@ -91,7 +91,7 @@ def fixture_board() -> Board:
 
 def make_game(
     players: int = 2,
-    config: RulesConfig = VARIANT_A,
+    config: RulesConfig = DEFAULT_RULES,
     dice: Iterable[int] = (),
     deck: tuple[tuple[Card, int], ...] = NO_CARDS,
     board: Board | None = None,