Use turn_id_t (int32_t) for turn-count across simulation, network and replay

CSimulation2 used , CSimulation2Impl mixed /, and
CTurnManager used  for the same turn-count concept, forcing a
static_cast<int64_t> at the one place these types already met
(rejoin-test comparison).

Add  (alias for std::int32_t) in SimulationCommand.h and use
it consistently for every turn counter in CSimulation2/Impl,
CTurnManager and its subclasses (CLocalTurnManager,
CReplayTurnManager), CNetServerTurnManager/CNetClientTurnManager, and
the network wire format (m_Turn in CEndCommandBatchMessage,
CSimulationMessage, CSyncCheckMessage, CSyncErrorMessage, and
m_CurrentTurn in CLoadedGameMessage).

Turn *duration* fields (m_TurnLength, m_CommandDelay,
DEFAULT_TURN_LENGTH, COMMAND_DELAY_SP/MP, SetTurnLength,
GetSavedTurnLength return type) are left as u32 — they're milliseconds,
not a counter, and out of scope here.

Remaining turn_id_t vs size_t comparisons use std::cmp_equal or
explicit casts, matching the existing pattern in Simulation2.cpp.

Fixes #8718

source/simulation2/Simulation2.h

@@ -22,6 +22,7 @@
 #include "lib/file/vfs/vfs_path.h"
 #include "lib/status.h"
 #include "ps/Loader.h"
+#include "simulation2/helpers/SimulationCommand.h"
 #include "simulation2/system/DebugOptions.h"
 #include "simulation2/system/Entity.h"
 
@@ -240,7 +241,7 @@ public:
 	/**
 	 * Activate the rejoin-test feature for turn @param turn.
 	 */
-	void ActivateRejoinTest(int turn);
+	void ActivateRejoinTest(turn_id_t turn);
 
 	std::string GenerateSchema();