READMEs (both languages) describe the wide source envelope, the canonical
encoding and its fixed encoder parameters, and read_image's downscale
envelope; tool/config catalogs regenerate for the new schema and Config
fields. The read-image-dimension scenario now pins the acceptance the old
2000px admission cap refused: the 2001x1 source is admitted and stored
byte-identically, so the fixture stays platform-independent.