Zip archives are rejected again by the format check with a hint to unpack
them. Removes the zip reader, its tests and helper, the zip path in
loadFile and the zip-only layer renaming (finishLayer name, sourceName).
The archive message no longer suggests packing files as .zip. The
readStream test for always-ready streams moved to the parser tests.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>