Fix MSVC compiler and linker errors

- Fix struct/class forward declaration mismatches (JAIAudience, JASTrack) causing different MSVC mangled names and unresolved symbols
- Add jsystem_stubs.cpp with stubs for JASHeap, JASVoiceBank, J3DShapeTable, JAUSection, JHICommBuf, HIO/HIO2, JOR

src/d/actor/d_a_e_yr.cpp

@@ -1716,7 +1716,7 @@ static void e_yr_su_wait_move(e_yr_class* i_this) {
     }
 }
 
-f32 dummy() {
+static f32 dummy() {
     return -50.0f;
 }