Ensure a pondermove is always returned and tweak time management

Previously, if the search stopped after an aspiration fail-high, no ponder move could be extracted from the PV.
This patch allows using the TT as a fallback for these cases.
Also, the previous time management led to some time losses when pondering was enabled under short TCs. The strategy is updated to consider pondering time in the allocated search time.

Non-regression STC:
LLR:  2.95/2.94<-6.00, 0.00> Elo diff: 0.07 [-2.19, 2.34] (95%)
Games: 18782 W: 2444 L: 2440 D: 13898 Draw ratio: 74.0%
Pntl: [96, 1567, 6062, 1569, 97]

STC w/ pondering (both sides):
LLR:  2.97/2.94<0.00, 10.00> Elo diff: 45.64 [27.86, 63.67] (95%)
Games: 490 W: 123 L: 59 D: 308 Draw ratio: 62.9%
Pntl: [1, 36, 123, 68, 17]

No functional change

src/Search.cpp

@@ -53,7 +53,6 @@ namespace Search
     {
         m_timer = timer;
         m_managed = true;
-        m_movetime_ms = movetime_ms;
         m_optimum = std::numeric_limits<double>::infinity();
         m_pondering.store(ponder);
         m_end_time.store(m_timer.begin() + std::chrono::milliseconds(movetime_ms));
@@ -63,7 +62,6 @@ namespace Search
     {
         m_timer = timer;
         m_managed = true;
-        m_movetime_ms = movetime_ms;
         m_optimum = optimum_ms / 1000.0;
         m_pondering.store(ponder);
         m_end_time.store(m_timer.begin() + std::chrono::milliseconds(movetime_ms));
@@ -72,8 +70,6 @@ namespace Search
     void SearchTime::ponderhit()
     {
         m_pondering.store(false);
-        if (m_managed)
-            m_end_time.store(std::chrono::steady_clock::now() + std::chrono::milliseconds(m_movetime_ms));
     }
 
     bool SearchTime::pondering() const

src/Search.hpp

@@ -70,7 +70,6 @@ namespace Search
         Timer m_timer;
         bool m_managed;
         double m_optimum;
-        uint64_t m_movetime_ms;
         std::atomic_bool m_pondering;
         std::atomic<std::chrono::steady_clock::time_point> m_end_time;
 

src/Thread.cpp

@@ -504,6 +504,16 @@ void Thread::search()
         Move bestmove = *best_pv;
         Move pondermove = *(best_pv + 1);
 
+        // If we get no pondermove from the PV, use the TT to try to guess a move to ponder
+        if (pondermove == MOVE_NULL)
+        {
+            m_position.make_move(bestmove);
+            TranspositionEntry* entry = nullptr;
+            if (ttable.query(m_position.hash(), &entry) && m_position.board().legal(entry->hash_move()))
+                pondermove = entry->hash_move();
+            m_position.unmake_move();
+        }
+
         // Mandatory output to the GUI
         std::cout << "bestmove " << m_position.board().to_uci(bestmove);
         if (pondermove != MOVE_NULL)