Improve retrograde analisys and matefinding capability

fixes https://github.com/official-stockfish/Stockfish/issues/6328

Before calling search(depth), the patch ensures that depth is at least 1
whenever we encounter a decisive score in the transposition table (TT). This
prevents search(depth) from being executed by qsearch, which would otherwise
ignore that information.  Typically, decisive TT hits occur when analyzing a
mating sequence backward. Due to the nature of Iterative Deepening (IID), such
scores are usually first found at depth 0. Without this patch, valuable
information can be lost because qsearch may overwrite the TT entry by replacing
the value with a static evaluation, even though the node was already processed
at a higher depth. This is also why the engine sometimes loses track of an
already discovered mate.

Using ..\sf\patch.exe on matetrack.epd with --nodes 1000000
Engine ID:     Stockfish dev-20251015-nogit
Total FENs:    6554
Found mates:   3437
Best mates:    2438

Using ..\sf\master.exe on matetrack.epd with --nodes 1000000
Engine ID:     Stockfish dev-20251015-nogit
Total FENs:    6554
Found mates:   3337
Best mates:    2407

Passed STC
https://tests.stockfishchess.org/tests/view/68fa3fa7637acd2a11e72a79
LLR: 3.55 (-2.94,2.94) <0.00,2.00>
Total: 134144 W: 34960 L: 34471 D: 64713
Ptnml(0-2): 376, 14199, 37459, 14636, 402

Failed LTC
https://tests.stockfishchess.org/tests/view/68ffc1b5637acd2a11e73377
LLR: -3.10 (-2.94,2.94) <0.50,2.50>
Total: 75360 W: 19423 L: 19519 D: 36418
Ptnml(0-2): 38, 7553, 22605, 7435, 49

closes https://github.com/official-stockfish/Stockfish/pull/6386

bench: 2530552

src/search.cpp

@@ -1260,7 +1260,10 @@ moves_loop:  // When in check, search starts here
             (ss + 1)->pv[0] = Move::none();
 
             // Extend move from transposition table if we are about to dive into qsearch.
-            if (move == ttData.move && ttData.depth > 1 && rootDepth > 8)
+            // decisive score handling improves mate finding and retrograde analysis.
+            if (move == ttData.move
+                && ((is_valid(ttData.value) && is_decisive(ttData.value) && ttData.depth > 0)
+                    || (ttData.depth > 1 && rootDepth > 8)))
                 newDepth = std::max(newDepth, 1);
 
             value = -search<PV>(pos, ss + 1, -beta, -alpha, newDepth, false);