fix(cpu): PPCBUG-424+425 vmaddfp128/vmaddcfp128 operand swap + va128 field fix

PPCBUG-424: vmaddfp128 computed VA×VB+VD instead of ISA-mandated VA×VD+VB.
PPCBUG-425: vmaddcfp128 computed VD×VB+VA instead of ISA-mandated VA×VD+VB.

Root-cause discovered while writing the operand-order regression tests:
va128() was extracting PPC bits 6-10 (the same field as vd128's low 5 bits),
not PPC bits 11-15 where VA lives in VX128 form. This meant va128() silently
aliased vd128 for any instruction where VA != VD, making the operand swap
invisible in the existing denorm-flush test (which used VA == VD == v2).

Fixes in this commit:
- decoder.rs: va128() now extracts PPC bits 11-15 (host bits 20-16) + bit29.
  The vmx128_va128_uses_bit29 test encoding updated to match the correct field.
- interpreter.rs: vmaddfp128 changed from ai.mul_add(bi,di) to ai.mul_add(di,bi)
  (VA×VD+VB). vmaddcfp128 changed from di.mul_add(bi,ai) to ai.mul_add(di,bi).
  vmaddfp128_flushes_denormal_inputs redesigned with distinct VA/VD/VB registers
  (v1/v2/v3) so the flush test is independent of the accessor fix.
  New vmaddfp128_operand_order_va_times_vd_plus_vb and
  vmaddcfp128_operand_order_va_times_vd_plus_vb tests verify 2×3+10=16.
- disasm_goldens.rs + vmx128_registers.json: vmaddfp128/vmaddcfp128/vnmsubfp128
  golden raws updated to properly encode VA at PPC bits 11-15 (new raws:
  0x146328D4 / 0x14632914 / 0x14632954). vperm128 / vsrw128 golden operands
  updated to reflect correct VA extraction (v4 instead of v3/v0).

Affects all VMX128 binary ops that call va128(): vaddfp128, vsubfp128,
vmulfp128, vmaddfp128, vmaddcfp128, vnmsubfp128, vperm128, vsrw128 etc.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/xenia-cpu/src/decoder.rs

@@ -135,9 +135,9 @@ impl DecodedInstr {
 
     // VMX128 field extractors
 
-    /// VA128 (bits 6-10, plus bit from 29)
+    /// VA128 (bits 11-15, plus bit from 29)
     #[inline] pub fn va128(&self) -> usize {
-        (extract_bits(self.raw, 6, 10) | (extract_bits(self.raw, 29, 29) << 5)) as usize
+        (extract_bits(self.raw, 11, 15) | (extract_bits(self.raw, 29, 29) << 5)) as usize
     }
 
     /// VB128 (bits 16-20, plus bits from 28, 30)
@@ -1105,8 +1105,8 @@ mod tests {
 
     #[test]
     fn vmx128_va128_uses_bit29() {
-        // va128 = bits 6-10 + bit 29. va_lo = 7, bit 29 = 1 → va128 = 7 | 32 = 39.
-        let raw = (7u32 << (31 - 10)) | (1u32 << (31 - 29));
+        // va128 = bits 11-15 + bit 29. va_lo = 7, bit 29 = 1 → va128 = 7 | 32 = 39.
+        let raw = (7u32 << (31 - 15)) | (1u32 << (31 - 29));
         let d = DecodedInstr { opcode: PpcOpcode::Invalid, raw, addr: 0 };
         assert_eq!(d.va128(), 39);
     }