A transfer is kept in a key-value state as JSON, and its empty Chunks map is omitted, so the one
read back for the first chunk had none and Take panicked: every publish in parts through
BusTransfers. The unit tests' transfers now round-trip JSON as the bus does. Found by the forge
module's whole test (CAT-G3).
Claude-Session: https://claude.ai/code/session_01JG73KJNrH5EgFXMKKYSrig