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/tests/golden/extended_mnemonics.json

@@ -525,7 +525,9 @@
       "raw": "0x7C2004AC",
       "addr": "0x82000000",
       "mnemonic": "sync",
-      "operands": ""
+      "operands": "",
+      "ext_mnemonic": "lwsync",
+      "ext_operands": ""
     },
     {
       "label": "trap",