Resolve the manifest's path from a project config path and its build.source_path value: /.docuriser, where a relative source_path resolves against the config's directory and an empty one falls back to the config directory itself (the same anchoring build paths have always used).

Read + parse + format-check a manifest. Returns false with *error filled (when non-null) if the file is missing, unparseable, or declares a format this build can't read; the message names the manifest path.