[JK2] Fix crash/hang when loading saved games on 64-bit builds

[dpiers]

Summary

Fixes #1278 - OpenJO x64: Crash when loading saved game

Root Cause

The watchTarget field (gentity_t*) in gNPC_t was being serialized as int32_t in sg_export() but was not included in the savefields_gNPC pointer conversion table. This caused:

1. On save: 64-bit pointer truncated to 32-bit garbage
2. On load: Truncated value interpreted as an invalid pointer
3. On use: Crash or hang when NPC_BSCinematic() calls CalcEntitySpot(NPCInfo->watchTarget, ...)

Investigation

Using the crash dump and save file from issue #1278:

• Windows x64: Crashes with EXCEPTION_ACCESS_VIOLATION at 0x426e6fa8
• macOS ARM64: Hangs in infinite loop - process sampling showed:

  NPC_BSCinematic() → CalcEntitySpot() → ViewHeightFix()
  

  The game was stuck calling CalcEntitySpot(NPCInfo->watchTarget, SPOT_HEAD_LEAN, viewSpot) with a corrupted pointer at line 252 of NPC_behavior.cpp.

Fix

Add the missing watchTarget entry to savefields_gNPC in codeJK2/game/g_savegame.cpp:

{strNPCOFS(watchTarget),        F_GENTITY},

This ensures the pointer is properly converted to an entity index when saving and restored to a valid pointer when loading. Also - improve game save parameter validation to return null for invalid entity IDs to maintain save compatibility.

Notes

• The Jedi Academy codebase (code/game/g_savegame.cpp) correctly includes watchTarget in its savefields_gNPC table - this was an oversight specific to the JK2 codebase.
• Save files created with the unfixed 64-bit build are corrupted and cannot be recovered, as the original entity index was lost when overwritten with a truncated pointer.
• This bug was latent for years because 32-bit builds don't truncate pointers, so the raw pointer value would still work after reload.