fix(disasm): PPCBUG-641+649 sync/lwsync L-field discrimination

PPCBUG-641: PpcOpcode::sync emitted "sync" regardless of the L-field at
PPC bit 10. The Xbox 360 acquire barrier (encoding 0x7C2004AC, L=1) is
lwsync, used in every spinlock. The disassembly DB stored every lwsync
as `mnemonic='sync'`, so `SELECT WHERE mnemonic='lwsync'` returned zero
rows regardless of binary content.

PPCBUG-649 (companion): the golden fixture for lwsync had no ext_mnemonic
field, pinning the wrong output and defeating regression detection.

Fix: in disasm.rs, gate on `(instr.raw >> 21) & 1` (PPC bit 10) — when
set, emit the lwsync extended form. Update extended_mnemonics.json
fixture to expect `ext_mnemonic: "lwsync"`.

Note: this is the disassembler-side fix only. The interpreter-side
PPCBUG-088 (lwsync vs sync semantics) is separate.

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

crates/xenia-cpu/src/disasm.rs

@@ -361,7 +361,15 @@ pub fn format(instr: &DecodedInstr) -> DisasmText {
         PpcOpcode::dcbz  => fmt_cache(instr, "dcbz"),
         PpcOpcode::dcbz128 => fmt_cache(instr, "dcbz128"),
         PpcOpcode::icbi  => fmt_cache(instr, "icbi"),
-        PpcOpcode::sync  => base("sync", String::new(), 0),
+        PpcOpcode::sync  => {
+            // L-field at PPC bit 10 (host bit 21) selects lwsync (L=1), the
+            // acquire barrier in every Xbox 360 spinlock. PPCBUG-641.
+            if (instr.raw >> 21) & 1 == 1 {
+                with_ext("sync", String::new(), 0, "lwsync", String::new(), 0)
+            } else {
+                base("sync", String::new(), 0)
+            }
+        }
         PpcOpcode::eieio => base("eieio", String::new(), 0),
         PpcOpcode::isync => base("isync", String::new(), 0),