Errors

Every problem in a file is reported at once with a stable code and a location:

hero.px:14: E_UNKNOWN_KEY (frame walk/down/1, row 2, x=[3]): keys 'q' aren't in the palette

Every error line starts with the command (ellipse: E_BAD_ARG: ...), and one that fails on an input file also says which input:

compose: layer 2 (parts.px:hat): parts.px:4: E_ROW_WIDTH (frame hat, row 1): row is 1 wide, ...

A command that fails prints no notes or WARNINGs: they describe the write it was about to make, and nothing was written. A command that writes into a directory that doesn't exist makes it and says so: created out/.

Unknown @sections are kept as-is, or rejected with check --strict.