apu: register-level emulation of 0xFF10-0xFF3F (fixes music fades)

The APU was a black hole: reads returned 0xFF, writes were dropped. That hangs
any game that fades music out by decrementing the NR50/rAUDVOL volume register
until it reads 0 (Pokemon's StopMusic spin-wait) - the write never stuck, the
read never hit 0, infinite loop. Store the 48 sound registers and return them
with the standard hardware read-back OR-masks (NR50 mask 0x00 = exact readback).
No synthesis, just correct register behavior.

1 files changed, 5 insertions, 0 deletions

diff --git a/src/gb.h b/src/gb.h
index 806daba..7439615 100644
--- a/src/gb.h
+++ b/src/gb.h
@@ -94,6 +94,11 @@ struct GB {
     // serial
     u8 sb, sc;
 
+    // APU registers 0xFF10-0xFF3F. We don't synthesize sound, but games read
+    // these back (e.g. the music fade-out decrements NR50/rAUDVOL until it
+    // reads 0), so they must behave as real storage, not a 0xFF black hole.
+    u8 apu[0x30];
+
     // DMA
     u8 dma;