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.

provider/llamaswap/video.go

@@ -4,7 +4,6 @@ import (
 	"bytes"
 	"context"
 	"fmt"
-	"mime"
 	"mime/multipart"
 	"net/http"
 	"strconv"
@@ -139,18 +138,7 @@ func singleVideoResult(provider, model, verb string, raw []byte, contentType str
 // frame from its MIME subtype. The name is provider-chosen (never
 // caller-supplied), so no sanitization is needed.
 func initImageFilename(mimeType string) string {
-	mt := strings.ToLower(strings.TrimSpace(mimeType))
-	if parsed, _, err := mime.ParseMediaType(mt); err == nil {
-		mt = parsed
-	}
-	switch mt {
-	case "image/jpeg", "image/jpg":
-		return "frame.jpg"
-	case "image/webp":
-		return "frame.webp"
-	default: // unknown MIME — PNG is the safe hint
-		return "frame.png"
-	}
+	return imageFilename(mimeType, "frame")
 }
 
 // formatInt renders an optional int pointer for a form field; nil = "" (omit).