fix(llamaswap): a headerless non-image response was returned as a PNG

Gadfly on #23, blocking, 2/2 agreement — and it is the exact defect this
whole line of work has been about: a call that succeeds while handing back
the wrong bytes.

sniffImageMIME falls back to image/png when detection is inconclusive, and
the guard only consulted Content-Type. A response with NO Content-Type
therefore skipped the check entirely and was labelled a PNG. The shim answers
JSON on a semantic miss (no face found in the source or target), which is
precisely the body that would have sailed through as a successful image.

The check now validates the BYTES — http.DetectContentType must say image/ —
and the reported MIME prefers the server's own label only when that label is
itself an image type. Break-checked by restoring the header-only condition,
which fails the new test.

Also from that review:
  - index is documented as ignored under all=true, so a negative one is no
    longer rejected there; it is still rejected when it would actually be
    sent, and both halves are tested.
  - initImageFilename (video.go) was imageFilename with the base fixed to
    "frame" and now delegates to it — two copies of one extension table is
    how they drift.
  - DetectedFace carried Width/Height alongside Box, two sources of truth for
    one fact that can disagree after any transform. Now a Size() method
    derived from Box.
  - a dead `apiErr` in the test (declared, then `_ = apiErr`) was an
    abandoned errors.As check; it is wired up and now asserts callers can
    classify the error.
  - swapImg duplicated editInit verbatim; removed.

Not taken: adding a FaceSwapProvider/ModelOption surface to match the other
optional imagegen capabilities (single-model finding). There are no options
to carry yet, and inventing an empty option type to look symmetrical would be
API surface with nothing behind it. Worth revisiting when a real knob exists.

imagegen/faceswap.go

@@ -55,9 +55,13 @@ type DetectedFace struct {
 	Box [4]int
 	// Score is the detector's confidence, 0-1.
 	Score float64
-	// Width and Height are the box dimensions, carried so a caller can pick
-	// "the big face" without recomputing them.
-	Width, Height int
+}
+
+// Size returns the box dimensions. Derived rather than stored: carrying
+// width/height alongside Box is two sources of truth for one fact, and the
+// pair can disagree after any transform.
+func (f DetectedFace) Size() (w, h int) {
+	return f.Box[2] - f.Box[0], f.Box[3] - f.Box[1]
 }
 
 // FaceSwapper is the optional face-transfer surface. Separate interface so