Depth dependent reduction threshold when full-depth re-search

STC: https://tests.stockfishchess.org/tests/view/68894d577b562f5f7b73273b
LLR: 2.98 (-2.94,2.94) <0.00,2.00>
Total: 155072 W: 40651 L: 40150 D: 74271
Ptnml(0-2): 609, 18271, 39292, 18738, 626

LTC: https://tests.stockfishchess.org/tests/view/688c2705502b34dd5e71127a
LLR: 2.95 (-2.94,2.94) <0.50,2.50>
Total: 321012 W: 82891 L: 81995 D: 156126
Ptnml(0-2): 227, 34421, 90285, 35375, 198

This commit was generated using qwen3-235b-a22b-thinking-2507:

Prompt: https://rentry.co/iqtaoht7

Reasoning/thinking: https://rentry.co/wm6t9hye

closes https://github.com/official-stockfish/Stockfish/pull/6210

Bench: 2983938

src/search.cpp

@@ -1251,9 +1251,13 @@ moves_loop:  // When in check, search starts here
             if (!ttData.move)
                 r += 1139;
 
+            const int threshold1 = depth <= 4 ? 2000 : 3200;
+            const int threshold2 = depth <= 4 ? 3500 : 4600;
+
             // Note that if expected reduction is high, we reduce search depth here
             value = -search<NonPV>(pos, ss + 1, -(alpha + 1), -alpha,
-                                   newDepth - (r > 3200) - (r > 4600 && newDepth > 2), !cutNode);
+                                   newDepth - (r > threshold1) - (r > threshold2 && newDepth > 2),
+                                   !cutNode);
         }
 
         // For PV nodes only, do a full PV search on the first move or after a fail high,