D248: fix port-wide 30fps halving -- pointer-width scheduler ABI bug

osScAddClient() stashes a per-client "every retrace vs every other
retrace" flag in the next array slot's .next field (c[1].next = next);
__scHandleRetrace read it back via a hardcoded 8-byte offset that only
lands on the right memory when OSScClient is 8 bytes (32-bit
pointers). On this 64-bit port the struct is 16 bytes, so the read
landed inside the gfx client's own msgQ pointer (always non-NULL)
instead, silently collapsing the gfx client onto the audio client's
30Hz path even though the VI-retrace pacemaker itself was always
correctly ticking at 60Hz.

Fix indexes the struct (client[1].next) instead of hand-rolling the
byte offset, so it scales with pointer width on either platform --
same class as D122/D126/D132/D209, just in the scheduler instead of a
ROM-serialized struct. Verified via the Video.DisplayFPS overlay:
30 FPS pre-fix, 60 FPS post-fix, same build/scene (-level_09).
-level_09/-level_20 both crash-free post-fix.

src/sched.c

@@ -331,7 +331,24 @@ void __scHandleRetrace(OSSched *sc) {
             __scExec(sc, sp, dp);
     }
     for (client = sc->clientList; client != 0; client = client->next) {
+        /* D2xx (PC-port pointer-width fix): osScAddClient() stashes a
+         * per-client "every-other-frame" flag in the *next* array slot
+         * (`c[1].next = next`, see osScAddClient above) -- gfxClient gets
+         * NULL (send every retrace, 60Hz) and g_AudioClient gets a nonzero
+         * sentinel (send every other retrace, 30Hz). This read has to land
+         * on that same slot. The N64 line below finds it via a hardcoded
+         * 2*sizeof(s32)=8-byte offset, which only equals sizeof(OSScClient)
+         * (and thus &client[1]) when pointers are 32-bit; on this 64-bit
+         * port sizeof(OSScClient)==16, so the hardcoded offset instead reads
+         * the low 32 bits of client->msgQ itself (always non-NULL) and the
+         * gfx client silently loses its 60Hz path, halving to 30fps. Fix:
+         * index the real field instead of hardcoding the byte offset -- it
+         * naturally scales with the pointer width on either platform. */
+#ifdef PORT
+        if ((client[1].next == 0) || ((sc->frameCount & 1) == 0)) {
+#else
         if ((*((s32*)client + 2) == 0) || ((sc->frameCount & 1) == 0)) {
+#endif
             osSendMesg(client->msgQ, (OSMesg) &sc->retraceMsg, OS_MESG_NOBLOCK);
         }
     }