feat(faceswap): carry the measured outcome, not just the image

A face swap always returns an image and always looks like success. Whether the
likeness actually transferred is a different question, and until now nothing in
the response answered it — so a caller wanting to know went and asked a vision
model instead. That is wrong in precisely the cases that matter: shown a jogger
in a Georgetown cap holding McDonald's cups, a VLM answers "Bill Clinton"
whoever's face is on him. In the run that prompted this it reported failure on
six consecutive CORRECT swaps (measured afterwards at 0.79-0.84 cosine), and
the caller burned 21 minutes chasing a problem that did not exist.

Result.SwappedFaces now carries, per replaced face: pixel size, the target
image's dimensions, head yaw, and cosine similarity between the source face and
the face actually present in the output.

Yaw and FractionOfImage are the two that explain the complaint. The swap in
question replaced a 138px face in a 1010px-wide photo — 14% of the width,
correct and invisible at a glance — and elsewhere a face turned -82 degrees,
where the features carrying identity are edge-on and any swap reads as a
generic person. Same code on a 168px face in a 385px picture (44%, yaw 2) is
unmistakable. None of that was inferable from a bounding box.

Typed on Result rather than stuffed into Raw: a caller has to act on this, and
a value reachable only by type-asserting an `any` is one nobody finds in time.

doRawHeaders is doRaw with the whole header instead of only Content-Type; doRaw
delegates to it, so the other 25 call sites are untouched and there is still
one place where the status check and the size cap live.

A missing or malformed header yields nil, not an error — an older shim sends no
header, and a swap that produced a good image must not fail because the
diagnostics beside it were unreadable. Covered for absent/garbage/wrong-type,
and the parse is break-checked.

imagegen/imagegen.go

@@ -68,6 +68,15 @@ type Result struct {
 	// Images are the generated images, in the order the backend returned them.
 	Images []Image
 
+	// SwappedFaces is the measured outcome of a FaceSwap, one entry per face
+	// replaced. Empty for every other operation, and empty from a provider
+	// that does not measure. TYPED rather than tucked into Raw: a caller has
+	// to act on this — it is the only way to distinguish a swap that
+	// transferred the likeness from one that returned an image and nothing
+	// more — and a value reachable solely by type-asserting an `any` is one
+	// nobody discovers in time to use.
+	SwappedFaces []SwappedFace
+
 	// Raw is the provider-native response object, an escape hatch for
 	// provider-specific fields. May be nil; never required for normal use.
 	Raw any