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

TODO.md

@@ -30,6 +30,22 @@ P2: security: per-IP rate limiting on the image routes
 
 # Completed Steps
 
+- 2026-10-08 JPEG XL as an input and output format (part of #222): a source
+  whose bytes start with either JPEG XL signature, the bare codestream's `FF 0A`
+  or the container's, is detected as `image/jxl`, which the upstream fetch
+  accepts; `orig` of such a source is JPEG XL. The format `jxl` works in plain
+  and encrypted URLs and on the generator page, served as `image/jxl` and cached
+  like the other formats. `auto` chooses JPEG XL first when `Accept` names
+  `image/jxl`. govips sends libvips a JPEG XL distance with every save, so
+  libvips ignores the quality: pixa turns `q` into a distance with libvips' own
+  formula, keeping 100 lossy, and removes the metadata from the image before
+  saving, as govips cannot have libvips strip it from JPEG XL. libvips 8.16 and
+  later still write an EXIF block of their own into JPEG XL (orientation,
+  resolution, size, colour space, fixed defaults), with nothing from the source.
+  An image with an ICC profile is converted to sRGB before the JPEG XL save too,
+  and a CMYK image with none is converted to sRGB, as libvips cannot save CMYK
+  as JPEG XL. libvips' default effort, 7, is kept. JPEG XL is not yet the
+  default output.
 - 2026-10-08 requests no longer wait behind eviction queries that read a whole
   table (closes #227): the new `cache_usage` table holds the total cache usage,
   kept up to date by triggers on `source_content` and `variant_content` in the