Reads the DSL file, parses to AST, loads all --atlas-dir paths into
a registry, runs the builder, and writes pretty-printed JSON. The
output filename's basename becomes the map id when the DSL has no
'id' directive. Parser and builder errors are surfaced as
'<file>:<line>: <message>'. Also reorders the duplicate-atlas warning
so the atlas_id appears before the word 'duplicate'.
