German for 'quack', matching the Ente (German for 'duck') naming. All
references updated: package name, CLI binary, X-Client-Package header,
test descriptions, temp dir prefixes, README, Makefile docker tag.
test/client/usage.test.ts is a tutorial-as-test-suite that walks
through the entire quack API in order: login, whoami, listCollections,
listFiles, downloadFile, downloadThumbnail, toJSON/fromJSON, logout.
Each it() block is a self-contained example with prose commentary
explaining what the code does and why, with code samples showing the
API as a consumer would use it. The mock server performs real SRP and
crypto so the test data is structurally identical to production.
8 tests, all failing against the stub.
