- fixed: A sidedef's sector and linedef references were writable.

This cannot be allowed under any circumstances because this is what links the map geometry together.
Yes, it will break any mod that tries to play with these variables, but any such mod has to be considered broken by design and must be stopped.

wadsrc/static/zscript/mapdata.txt

@@ -57,9 +57,9 @@ struct Side native play
 		WALLF_LIGHT_FOG      = 128,	// This wall's Light is used even in fog.
 	};
 
-	native Sector	sector;			// Sector the SideDef is facing.
+	native readonly Sector sector;			// Sector the SideDef is facing.
 	//DBaseDecal*	AttachedDecals;	// [RH] Decals bound to the wall
-	native Line		linedef;
+	native readonly Line linedef;
 	native int16	Light;
 	native uint8	Flags;