feat(mapper-199): align bank_values_ext init defaults with Mesen2/FCEUX

Initialize CHR slots 1 and 3 (driven by exReg[2] / exReg[3], stored
in bank_values_ext[2] / bank_values_ext[3]) to bank values 1 and 3
respectively at construction time. Without this, the slots default
to bank 0 -- so any CHR read between Txrom::new() and the game's
$8001 init writes would render slots 1 and 3 from CHR-RAM bank 0
instead of banks 1 and 3.

In practice every observed mapper-199 ROM (Sangokushi II, Dragon
Ball Z 2 / Gaiden, etc.) overwrites these registers immediately
during boot via ctrl0=A and ctrl0=B writes, so this is defensive
alignment, not a gameplay fix. If post-fix testing shows behavior
is unchanged, this commit can be squashed into the mapper 199
feature commit.

Cross-verified against the references:
  * Mesen2 (Mmc3Variants/MMC3_199.h InitMapper):
      _exRegs[2] = 1; _exRegs[3] = 3;
  * FCEUX (boards/199.cpp M199Power):
      EXPREGS[2] = 1; EXPREGS[3] = 3;

Signed-off-by: Kefu Chai <tchaikov@gmail.com>

Author: tchaikov

tetanes-core/src/mapper/m004_txrom.rs

@@ -145,11 +145,16 @@ impl Txrom {
         };
         let last_bank = txrom.prg_rom_banks.last();
         if txrom.mapper_num == 199 {
-            // TypeG default: exPrg[2]=last-1 ($C000), exPrg[3]=last ($E000). These are the same
-            // as the standard MMC3 fixed-bank defaults, but they're written via exPrg so we init
-            // bank_values_ext accordingly. The game will overwrite via ctrl0=8 and ctrl0=9 writes.
+            // TypeG hardware power-on defaults:
+            //   exPrg[2]=last-1 ($C000), exPrg[3]=last ($E000) -- same as standard MMC3 fixed
+            //     banks, but written via exPrg in TypeG.
+            //   exChr[1]=1, exChr[3]=3 -- CHR slots 1 and 3 default to CHR-RAM banks 1 and 3
+            //     (banks < 8 redirect to ext_vram). The game overwrites these via ctrl0=A/B,
+            //     but defaults must be sane in case anything reads CHR before init.
             txrom.regs.bank_values_ext[0] = (last_bank - 1) as u8; // exPrg[2]
             txrom.regs.bank_values_ext[1] = last_bank as u8;       // exPrg[3]
+            txrom.regs.bank_values_ext[2] = 1;                     // exChr[1]
+            txrom.regs.bank_values_ext[3] = 3;                     // exChr[3]
         }
         txrom.prg_rom_banks.set(2, last_bank - 1);
         txrom.prg_rom_banks.set(3, last_bank);