Balance: easy AI stays smaller; balance runs documented

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/balance.rs

@@ -89,7 +89,12 @@ fn seeds() -> Vec<u32> {
 fn balance_difficulty() {
     println!("\n== difficulty duels (2 masters, 112x112, no waves, 45 min) ==");
     println!("{:<14} {:>5} {:>5} {:>5} {:>8}", "pair", "A", "B", "draw", "min");
-    for (a, b) in [(0u8, 1u8), (1, 2), (0, 2), (1, 1)] {
+    // BALANCE_PAIR=a,b runs one pairing only
+    let pairs: Vec<(u8, u8)> = match std::env::var("BALANCE_PAIR").ok().and_then(|p| p.split_once(',').map(|(a, b)| (a.parse().ok(), b.parse().ok()))) {
+        Some((Some(a), Some(b))) => vec![(a, b)],
+        _ => vec![(0, 1), (1, 2), (0, 2), (1, 1)],
+    };
+    for (a, b) in pairs {
         let (mut wa, mut wb, mut dr, mut mins) = (0, 0, 0, 0.0);
         for (k, seed) in seeds().into_iter().enumerate() {
             let mut g = arena(seed, 112);