feat(arch): add the VEX encoder to the extension layer

Assisted-by: GLM 5.3 Flash

arch/arm64_ext.go

@@ -1033,6 +1033,16 @@ type ExtInstr struct {
 	SveMode  uint8          // extMemNone or one of the extMem* modes below
 	SveShift int            // the required <<s, or the access-size shift
 	SveBase  ExtArrangement // the vector base's element size, where it has one
+	// Vex records that the amd64 entry encodes through the two-byte VEX
+	// prefix the AVX extensions carry, not through EVEX: a family with no
+	// AVX-512 form, the AVX-VNNI-INT16 dot products, lays its C4 word over
+	// five bytes, R/X/B inverted and the five-bit map in byte one, W and
+	// vvvv complemented and L and pp in byte two.  A VEX row takes no mask,
+	// broadcast or rounding decoration and no register above 15, and its
+	// memory forms keep the classic displacement choices no VEX word
+	// scales.  Every existing entry carries the zero value and encodes
+	// through EVEX.
+	Vex bool
 }
 
 // The arm64 gather/scatter addressing modes, the values SveMode carries.