Fix HGSS box saving (empty->not)

HGSS tracks each box if it is changed, to skip writing it on in-game save. PKHeX (and other editors) weren't updating these bitflags, so when empty boxes had Pokémon added and saved once in-game, the save file would corrupt as it was copying the checksum but not the box's data.

Saving twice in-game will cause the checksums to get updated even without the flags being set. I don't think we understand it fully, but this hacky workaround of noting all boxes as "changed" will cause the first-save to properly copy all contents of the boxes to the new primary save section. Rather than tracking which boxes we are reading/writing slots for, or fixing/comparing against a backup -- we need to account for API usage where people might write directly into the save (pcdata.bin/boxdata.bin too). Best to play it safe and let the game fix it.

Most wouldn't have noticed this issue as the box they're modifying is usually their most-recently-changed in-game box.

Closes #4368

PKHeX.Core/Saves/SAV4HGSS.cs

@@ -53,6 +53,14 @@ public sealed class SAV4HGSS : SAV4, IBoxDetailName, IBoxDetailWallpaper
         GetSAVOffsets();
     }
 
+    protected override byte[] GetFinalData()
+    {
+        // Make sure all boxes are copied when saved only once in-game.
+        // This results in the game "saving a lot of data", but ensures the boxdata struct does not corrupt in-game on single save.
+        FlagsBoxContentChanged = FlagsBoxContentChangedAll;
+        return base.GetFinalData();
+    }
+
     private const int OffsetMystery = 0x9D3C; // Flags and Gifts
     protected override int EventWork => 0xDE4;
     protected override int EventFlag => 0x10C4;
@@ -125,12 +133,17 @@ public sealed class SAV4HGSS : SAV4, IBoxDetailName, IBoxDetailWallpaper
         set => Storage[BOX_FLAGS] = value[0];
     }
 
-    public int Counter
+    /// <summary>
+    /// The box structure stores bitflags to indicate which boxes have changed; used when saving to skip unchanged boxes.
+    /// </summary>
+    public int FlagsBoxContentChanged
     {
         get => ReadInt32LittleEndian(Storage[(BOX_END + 4)..]);
         set => WriteInt32LittleEndian(Storage[(BOX_END + 4)..], value);
     }
 
+    private const int FlagsBoxContentChangedAll = 0x3_FFFF; // 18 boxes.
+
     private Span<byte> GetBoxNameSpan(int box) => Storage.Slice(GetBoxNameOffset(box), BOX_NAME_LEN);
     public string GetBoxName(int box) => GetString(GetBoxNameSpan(box));