feat(sim): cash_out_threshold — rathole stack excess to bankroll

After each hand, stack above the threshold moves to the bankroll.
Bounds effective depth while keeping natural short-stack dynamics
(losses, rebuy cycles). Needed for realistic deep-stack-safe
benchmarks: the oracle hybrid's EV collapses with stack depth
(+6.9 at 75BB reset vs -33.8 natural), so unbounded natural-stack
snowballs distort candidate evaluation.

holdem_core/src/game/config.rs

@@ -354,6 +354,12 @@ pub struct GameConfig {
     #[serde(default)]
     pub bankroll_refill_threshold: ChipCount,
 
+    /// Cash-out (rathole) threshold: after each hand, stack excess above
+    /// this amount is moved to the player's bankroll. 0 = disabled.
+    /// Keeps natural stack dynamics (losses, rebuys) while bounding depth.
+    #[serde(default)]
+    pub cash_out_threshold: ChipCount,
+
     /// If true, player seat assignments are randomized before each hand
     /// (cash game) or before tournament start.
     #[serde(default)]
@@ -430,6 +436,7 @@ impl Default for GameConfig {
             rake_cap: 0,
             reset_bankroll_each_hand: false,
             bankroll_refill_threshold: 0,
+            cash_out_threshold: 0,
             randomize_seats: false,
             auto_rebuy: false,
             rebuy_threshold_percentage: default_rebuy_threshold(),
@@ -1069,6 +1076,7 @@ min_raise = 50
             rake_cap: 0,
             reset_bankroll_each_hand: false,
             bankroll_refill_threshold: 0,
+            cash_out_threshold: 0,
             randomize_seats: false,
             auto_rebuy: false,
             rebuy_threshold_percentage: 25,
@@ -1158,6 +1166,7 @@ min_raise = 50
             rake_cap: 0,
             reset_bankroll_each_hand: false,
             bankroll_refill_threshold: 0,
+            cash_out_threshold: 0,
             randomize_seats: false,
             auto_rebuy: false,
             rebuy_threshold_percentage: 25,
@@ -1197,6 +1206,7 @@ min_raise = 50
             rake_cap: 0,
             reset_bankroll_each_hand: false,
             bankroll_refill_threshold: 0,
+            cash_out_threshold: 0,
             randomize_seats: false,
             auto_rebuy: false,
             rebuy_threshold_percentage: 25,
@@ -1252,6 +1262,7 @@ min_raise = 50
             rake_cap: 0,
             reset_bankroll_each_hand: false,
             bankroll_refill_threshold: 0,
+            cash_out_threshold: 0,
             randomize_seats: false,
             auto_rebuy: false,
             rebuy_threshold_percentage: 25,
@@ -1309,6 +1320,7 @@ min_raise = 50
             rake_cap: 0,
             reset_bankroll_each_hand: false,
             bankroll_refill_threshold: 0,
+            cash_out_threshold: 0,
             randomize_seats: false,
             auto_rebuy: false,
             rebuy_threshold_percentage: 25,
@@ -1360,6 +1372,7 @@ min_raise = 50
             rake_cap: 0,
             reset_bankroll_each_hand: false,
             bankroll_refill_threshold: 0,
+            cash_out_threshold: 0,
             randomize_seats: false,
             auto_rebuy: false,
             rebuy_threshold_percentage: 25,
@@ -1422,6 +1435,7 @@ min_raise = 50
             rake_cap: 0,
             reset_bankroll_each_hand: false,
             bankroll_refill_threshold: 0,
+            cash_out_threshold: 0,
             randomize_seats: false,
             auto_rebuy: false,
             rebuy_threshold_percentage: 25,
@@ -1475,6 +1489,7 @@ min_raise = 50
             rake_cap: 0,
             reset_bankroll_each_hand: false,
             bankroll_refill_threshold: 0,
+            cash_out_threshold: 0,
             randomize_seats: false,
             auto_rebuy: false,
             rebuy_threshold_percentage: 25,
@@ -1594,6 +1609,7 @@ min_raise = 20
                 rake_cap: 0,
                 reset_bankroll_each_hand: false,
                 bankroll_refill_threshold: 0,
+                cash_out_threshold: 0,
                 randomize_seats: false,
                 auto_rebuy: false,
                 rebuy_threshold_percentage: 25,

holdem_core/src/game/simulation.rs

@@ -128,6 +128,28 @@ pub fn run_cash_game(config: &GameConfig) -> Result<(SimulationStats, Table), Ga
                         }
                     }
                 }
+                // Cash-out (rathole): move stack excess above the threshold
+                // into the bankroll after each hand. Bounds effective depth
+                // while keeping natural short-stack dynamics intact.
+                if config.cash_out_threshold > 0 {
+                    for slot in table.seats_mut().iter_mut() {
+                        if let Some(seat) = slot {
+                            if seat.stack > config.cash_out_threshold {
+                                let excess =
+                                    seat.stack - config.cash_out_threshold;
+                                log::debug!(
+                                    "Cash-out: {} ratholes {} (stack {} -> {})",
+                                    seat.player.name(),
+                                    excess,
+                                    seat.stack,
+                                    config.cash_out_threshold
+                                );
+                                seat.bankroll += excess as i64;
+                                seat.stack = config.cash_out_threshold;
+                            }
+                        }
+                    }
+                }
             }
             Err(GameError::NotEnoughPlayers(count)) => {
                 // Not enough active players to continue. This should only
@@ -175,7 +197,9 @@ pub fn run_cash_game(config: &GameConfig) -> Result<(SimulationStats, Table), Ga
                 .map(|seat| (seat.player_id, seat.bankroll))
         })
         .collect();
-    let bankroll_mode = config.reset_bankroll_each_hand || config.bankroll_refill_threshold > 0;
+    let bankroll_mode = config.reset_bankroll_each_hand
+        || config.bankroll_refill_threshold > 0
+        || config.cash_out_threshold > 0;
     stats.record_final_stacks(&final_stacks, &starting_stacks, &bankrolls, bankroll_mode, config.auto_rebuy);
 
     Ok((stats, table))