Route JPEG XL through the native libvips path (govips jxlload) like
HEIC/AVIF, keeping the external "djxl" decoder as an automatic fallback
for runtimes whose libvips lacks JPEG XL support.
Decouple format enablement from "djxl": DisableJpegXL now stays enabled
when either the decoder is present or libvips can decode JPEG XL natively.
The libvips capability probe (thumb.JpegXLSupported) is cached and runs
only when no decoder is present, so config introspection on standard
installs (which ship "djxl") does not start libvips.
Add a shared assets/samples/dice.jxl fixture and focused tests for the
native conversion path, the djxl fallback command, and the capability
decision table.
