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>

RULES.md

@@ -1,4 +1,4 @@
-# Il Labirinto del Dojo — Rules (draft v0.5)
+# Il Labirinto del Dojo — Rules (draft v0.7)
 
 Working language: English. The final gift edition will be produced separately in Italian.
 
@@ -34,8 +34,6 @@ Each player has a position from **0 to 17**. Your belt is the segment you are in
 
 - One **stripe** = 1 step on the track. One **belt** = 3 steps.
 - Position never goes below 0 or above 17.
-- **[VARIANT]** *Brown requires Kumite*: squares and cards cannot move a player from 14 to 15.
-  Once Brown, squares and cards may move the player freely within 15–17.
 - A player below 15 is not Brown, whatever their previous belt was.
 
 ## Setup
@@ -93,7 +91,7 @@ Each player has a position from **0 to 17**. Your belt is the segment you are in
 |---|---|---|
 | Equal belts | +3 | −3, sent to Start |
 | Lower belt wins (upset) | +3 | −3, sent to Start |
-| Higher belt wins | **[VARIANT]** 0 or +1 | sent to Start |
+| Higher belt wins | nothing | sent to Start |
 
 ## Squares
 
@@ -159,23 +157,19 @@ All other cards are shown and resolved immediately.
   Meditating players cannot defend; if nobody can, you return to the square you entered
   from and there is no Exam this turn.
 - The exam is a Kumite with the challenger as attacker; Senshu and Tecnica cards apply.
+- **The defender fights as a Black belt**, whatever their own belt: the Brown challenger is
+  one belt lower, so the defender adds +1.
   - **Challenger wins**: they wear the Black Belt and win the game.
   - **Challenger loses**: −3 and sent to Start. The defender gains nothing.
   - If the challenger plays Uke and loses, they return to the square they entered from instead of Start.
 
 ## Rule variants under test
 
-| Variant | Brown requires Kumite | Higher belt beats lower |
-|---|---|---|
-| **A** | No | winner gains 0 |
-| **B** | Yes (squares/cards cannot move 14 → 15) | winner gains +1 |
-
-Rationale for B: with the cap, a Blue leader on 14 facing only lower belts could never
-become Brown if beating lower belts gave nothing. The +1 removes that deadlock.
-
-Other parameters to validate by simulation: Kiai +2 vs +1; starting on 3 with 6+ players;
+Parameters to validate by simulation: Kiai +2 vs +1; starting on 3 with 6+ players;
 square mix; game length at 3/4/6/8 players.
 
+Tested and set aside (simulator switches only): Exam ties to the defender; best-of-3 Exam.
+
 ## FAQ
 
 **Which effects can I refuse?**