Wrap depth reductions in DepthReduction struct

This lets us easily encapsulate the fact that the reduction should not
be less than zero, and give nice names to decreasing and increasing the
reduction.

Ran a few thousand games at STC with negligible Elo diff:

Elo   | 1.91 +- 8.72 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=16MB
LLR   | 0.61 (-2.94, 2.94) [-5.00, 0.00]
Games | N: 2732 W: 752 L: 737 D: 1243
Penta | [60, 346, 557, 325, 78]

src/engine/search/negamax.rs

@@ -11,6 +11,27 @@ use crate::engine::search::time_control::TimeStrategy;
 use crate::engine::search::transposition::{NodeBound, SearchTranspositionTableData};
 use crate::engine::tablebases::Wdl;
 
+pub struct DepthReduction(u8);
+
+impl DepthReduction {
+    #[inline]
+    #[expect(unused)]
+    pub fn increase_if(&mut self, predicate: bool) {
+        self.0 = self.0.saturating_add(predicate as u8);
+    }
+
+    #[inline]
+    #[expect(unused)]
+    pub fn reduce_if(&mut self, predicate: bool) {
+        self.0 = self.0.saturating_sub(predicate as u8);
+    }
+
+    #[inline]
+    pub fn value(&self) -> u8 {
+        self.0
+    }
+}
+
 pub fn negamax(
     game: &mut Game,
     mut alpha: Eval,
@@ -205,7 +226,8 @@ pub fn negamax(
                 && number_of_legal_moves >= params::LMR_MOVE_THRESHOLD
                 && !in_check
             {
-                lmr_reduction(depth, number_of_legal_moves)
+                let reduction = DepthReduction(lmr_reduction(depth, number_of_legal_moves));
+                reduction.value()
             } else {
                 1
             };