Power-on OBP0/OBP1 read 0xff; missing cart RAM reads float to 0xff

Judge_'s gbtests INITREGS samples FF40-FF4F at boot: the object
palettes are uninitialized on hardware and read back 0xff (neither boot
ROM writes them), and reads from the cartridge RAM window with no RAM
present float to 0xff rather than 0. With both fixed the suite passes
INITREGS, all TIM*, IRQTIM, LYCSTAT0-3, STATTIM and STATTIMS1-4;
only STATTIMS5 remains (one STAT sample where our mode 3 ends a machine
cycle earlier than hardware - the known mode-3 boundary tension
documented in CLAUDE.md). Refs #64.

Full battery green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J1wLWscyUGS7CFwc3CjJR8

Author: trekawek

core/src/main/java/eu/rekawek/coffeegb/core/gpu/GpuRegisterValues.java

@@ -37,6 +37,10 @@ public GpuRegisterValues() {
         values = new int[GpuRegister.values().length];
         java.util.Arrays.fill(mixValues, -1);
         java.util.Arrays.fill(pendingMixValues, -1);
+        // the object palettes are uninitialized at power on and read 0xff; neither
+        // boot ROM writes them (gbtests INITREGS)
+        values[GpuRegister.OBP0.ordinal()] = 0xff;
+        values[GpuRegister.OBP1.ordinal()] = 0xff;
     }
 
     public int get(GpuRegister reg) {

core/src/main/java/eu/rekawek/coffeegb/core/memory/cart/type/BasicRom.java

@@ -24,9 +24,10 @@ public void setByte(int address, int value) {
     @Override
     public int getByte(int address) {
         if (address >= 0x0000 && address < 0x8000) {
-            return rom[address];
+            return address < rom.length ? rom[address] : 0xff;
         } else {
-            return 0;
+            // no cartridge RAM: reads float to 0xff (gbtests INITREGS)
+            return 0xff;
         }
     }