fix(disasm): name the amd64 families x/arch decodes to the zero opcode

x/arch reports the ADCX, ADOX, RDSEED, RDPID, TPAUSE, UMONITOR, UMWAIT
and ENDBR families with no error but the degenerate zero instruction,
which GoSyntax renders as Op(0) under its prefix decoration and with a
length of one.  A supplementary naming table keyed by the opcode
pattern restores the toolchain's own spellings and lengths; the parity
fixtures pin all 41 corpus rows (ENDBR32 alone, which the toolchain
cannot spell, pins as bytes and text in the focused naming test).

Assisted-by: GLM 5.3 Flash

disasm/disasm.go

@@ -70,6 +70,16 @@ func Decode(a arch.Arch, code []byte, addr uint64) (Instruction, error) {
 		if err != nil {
 			return Instruction{Addr: addr, Text: "???", Len: 1}, nil
 		}
+		if inst.Op == 0 {
+			// x/arch reports a few opcode families with no error but the
+			// degenerate zero instruction: no opcode, no operands and a
+			// length of one, which GoSyntax renders as "Op(0)".  The
+			// supplementary naming table restores the families the Go
+			// toolchain names; anything else keeps the placeholder.
+			if text, n, ok := nameAMD64Degenerate(code); ok {
+				return Instruction{Addr: addr, Text: text, Len: n}, nil
+			}
+		}
 		return Instruction{Addr: addr, Text: x86asm.GoSyntax(inst, addr, nil), Len: inst.Len}, nil
 	}
 }