A file-watching version of Chapter 5's duplicate finder. The current Dup_Finder scans once and returns. Watching a directory continuously and re-scanning only what changed is a genuinely different design problem — incremental state instead of a clean one-shot pass — and deserved its own chapter more than a paragraph tacked onto Part III.
A second Java toolkit, to test whether Chapter 8's lesson generalizes. This book crosses into javax.swing exactly once. Whether the same isolation pattern — capture events in plain Nex state, render separately — holds for a different host library (a database driver, a different UI toolkit, an existing Java service client) is an open question this book does not answer, on purpose: one crossing, examined carefully, teaches the discipline better than several crossings examined thinly.
Authentication and authorization on Chapter 7's HTTP API. calc_service trusts every request. A real API boundary — API keys, rate limiting, a contract about who is allowed to call what — is a natural extension of the same chapter's "parse, don't validate" argument, applied to a caller's identity instead of a request body, and was left out to keep that chapter's focus on the translation layer between a language-level contract and an HTTP status code.
A persistent version of Chapter 6's chat server. Messages vanish the moment the hub task exits. Logging them — reusing Chapter 2's schema-versioning discipline for a message history instead of a task list — is a natural bridge between two parts of this book that never talk to each other otherwise.
Backpressure, for real, somewhere. Chapter 5's cancellation illustration is deliberately separate from its actual scan; nothing in this book has a channel or a task queue that can genuinely fill up and has to decide what to do about it. A producer that outpaces its consumer — a log ingester, a bounded work queue — would put a real bounded Channel under real pressure, which none of these nine projects ever do.
None of these needed to exist for this book's argument to hold — contracts, stated before code, holding across a CLI, a library, concurrency, a network, and a GUI. They are left here as an invitation rather than a promise: the same five-part anatomy (Problem, Design as Contract, Build, Test, Takeaways) that structured every chapter in this book works just as well on a project nobody has written the prose for yet.