let ScreenMsg take a format string it will not write to

STR16 is CHAR16*, so ScreenMsg asked for a mutable format string it has no
intention of mutating: the body only hands it to va_start and vswprintf, and
vswprintf takes it as const.

Calls that pass a literal are accepted anyway under the MSVC rules clang-cl
applies. Calls that pass anything else are not, and DynamicDialogue.cpp picks
the format at runtime:

    ScreenMsg( FONT_MCOLOR_LTGREEN, MSG_INTERFACE,
               ...sOpinionModifier >= 0 ? L"%s: %s +%d" : L"%s: %s %d", ... );

The conditional has type const wchar_t*, and there is nowhere for it to go.

Declaring the parameter const CHAR16* is what the function always meant. Every
existing caller still compiles: a literal and a CHAR16* both convert.

Verification:

    ninja -C build parse         # DynamicDialogue.cpp clean
    ninja -C build -k 0          # Release, four applications, green
    ninja -C build-debug -k 0    # Debug, four applications, green

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Utils/message.cpp

@@ -610,7 +610,7 @@ void UnHideMessagesDuringNPCDialogue( void )
 	return;
 }
 
-void ScreenMsg( UINT16 usColor, UINT8 ubPriority, STR16 pStringA, ...)
+void ScreenMsg( UINT16 usColor, UINT8 ubPriority, const CHAR16* pStringA, ...)
 {
 	//__try
 	//{

Utils/message.h

@@ -55,7 +55,7 @@ extern BOOLEAN fDisableJustForIan;
 // are we allowed to beep on message scroll in tactical
 extern BOOLEAN fOkToBeepNewMessage;
 
-void ScreenMsg( UINT16 usColor, UINT8 ubPriority, STR16 pStringA, ...);
+void ScreenMsg( UINT16 usColor, UINT8 ubPriority, const CHAR16* pStringA, ...);
 
 // same as screen message, but only display to mapscreen message system, not tactical
 void MapScreenMessage( UINT16 usColor, UINT8 ubPriority, STR16 pStringA, ...);