Fix unhandled IRQ on IIGS

src/libsrc/apple/mouse.pla

@@ -50,6 +50,10 @@ asm updateMouse
         LDY     #$40
         JMP     $C400           ; IIGS REQUIRES THIS HAPPEN IN IRQ
 end
+asm serviceCYA#0
+        CLC
+        RTS
+end
 //
 // Check for VBL (timer) and Mouse events (atomic read and reset)
 //
@@ -175,6 +179,13 @@ for rom = $C100 to $C700 step $0100
         params:2 = @serviceMouse
         syscall($40, @params)
         //
+        // Hook CYA IRQ handler into ProDOS IRQ chain
+        //
+        params.0 = 2
+        params.1 = 3
+        params:2 = @serviceCYA
+        syscall($40, @params)
+        //
         // Set values
         //
         slot           = rom >> 8