Serve and accept JPEG XL as an image format (part of #222)

A JPEG XL source is accepted, and orig of one is JPEG XL. The format
jxl works in plain and encrypted URLs and on the generator page,
served as image/jxl; auto chooses it first when Accept names
image/jxl.

govips sends libvips a JPEG XL distance, which overrides the quality,
so q is turned into a distance, keeping 100 lossy. govips cannot have
libvips strip metadata from JPEG XL, so it is removed from the image
before saving; libvips 8.16 and later still add an EXIF block of their
own, with nothing from the source. The conversion to sRGB moved ahead
of the format switch, and a CMYK image with no ICC profile is
converted to sRGB too, as libvips cannot save CMYK as JPEG XL.

Model: opus-5-5

internal/magic/magic.go

@@ -26,6 +26,7 @@ const (
 	MIMETypeWebP = MIMEType("image/webp")
 	MIMETypeGIF  = MIMEType("image/gif")
 	MIMETypeAVIF = MIMEType("image/avif")
+	MIMETypeJXL  = MIMEType("image/jxl")
 	MIMETypeSVG  = MIMEType("image/svg+xml")
 )
 
@@ -40,6 +41,7 @@ const (
 	FormatPNG      ImageFormat = "png"
 	FormatWebP     ImageFormat = "webp"
 	FormatAVIF     ImageFormat = "avif"
+	FormatJXL      ImageFormat = "jxl"
 	FormatGIF      ImageFormat = "gif"
 )
 
@@ -62,6 +64,11 @@ var (
 	// AVIF uses the ftyp box with brand "avif" or "avis"
 	// Format: size(4 bytes) + "ftyp" + brand(4 bytes)
 	magicFtyp = []byte{0x66, 0x74, 0x79, 0x70} // "ftyp"
+	// JPEG XL is a bare codestream or a codestream in a container
+	magicJXLCodestream = []byte{0xFF, 0x0A}
+	magicJXLContainer  = []byte{
+		0x00, 0x00, 0x00, 0x0C, 0x4A, 0x58, 0x4C, 0x20, 0x0D, 0x0A, 0x87, 0x0A,
+	}
 )
 
 // WebP identifier appears at offset 8 after RIFF header.
@@ -115,6 +122,12 @@ func DetectFormat(data []byte) (MIMEType, error) {
 		}
 	}
 
+	// Check JPEG XL (FF0A, or the 12-byte container signature)
+	if bytes.HasPrefix(data, magicJXLCodestream) ||
+		bytes.HasPrefix(data, magicJXLContainer) {
+		return MIMETypeJXL, nil
+	}
+
 	// Check SVG - look for XML declaration or SVG tag
 	if detectSVG(data) {
 		return MIMETypeSVG, nil
@@ -181,7 +194,8 @@ func normalizeMIMEType(mimeType string) string {
 func IsSupportedMIMEType(mimeType string) bool {
 	normalized := normalizeMIMEType(mimeType)
 	switch MIMEType(normalized) {
-	case MIMETypeJPEG, MIMETypePNG, MIMETypeWebP, MIMETypeGIF, MIMETypeAVIF, MIMETypeSVG:
+	case MIMETypeJPEG, MIMETypePNG, MIMETypeWebP, MIMETypeGIF, MIMETypeAVIF,
+		MIMETypeJXL, MIMETypeSVG:
 		return true
 	default:
 		return false
@@ -226,6 +240,8 @@ func MIMEToImageFormat(mimeType string) (ImageFormat, bool) {
 		return FormatGIF, true
 	case MIMETypeAVIF:
 		return FormatAVIF, true
+	case MIMETypeJXL:
+		return FormatJXL, true
 	case MIMETypeSVG:
 		// SVG has no corresponding output format.
 		return "", false
@@ -247,6 +263,8 @@ func ImageFormatToMIME(format ImageFormat) string {
 		return string(MIMETypeGIF)
 	case FormatAVIF:
 		return string(MIMETypeAVIF)
+	case FormatJXL:
+		return string(MIMETypeJXL)
 	case FormatOriginal:
 		// Original format passes content through unchanged.
 		return mimeOctetStream