feat(faceswap): expose yaw on enumeration too

ListFaces now carries head yaw, so a caller choosing WHICH face to swap can
see the thing that decides whether the swap will read — not only learn it
afterwards from the swap report. In the run that prompted this the target's
three faces sat at -82, -8 and -11 degrees; only the first was hopeless, and
nothing in a bounding box said so.

imagegen/faceswap.go

@@ -55,6 +55,14 @@ type DetectedFace struct {
 	Box [4]int
 	// Score is the detector's confidence, 0-1.
 	Score float64
+
+	// Yaw is how far the head is turned from camera, in degrees, or nil when
+	// the provider does not report pose. Exposed on ENUMERATION, not just
+	// after the fact, because it is how a caller picks a face a swap will
+	// actually work on: past roughly ±45° the features carrying identity are
+	// edge-on, and the result reads as a generic person however good the
+	// transfer is. A bounding box cannot show this.
+	Yaw *float64
 }
 
 // Size returns the box dimensions. Derived rather than stored: carrying