FFI error, progress, and cancellation

The error taxonomy, the progress stream, and cooperative cancellation of a long-running operation are one contract. Design them together. Separate designs give an error enum that cannot say "the user pressed Cancel", a progress stream that leaks a worker when the consumer goes away, and a cancel call that blocks the UI thread.

Related skills, when they are installed:

• uniffi-boundary: binding mechanics, derives, and the full ProgressListener foreign-trait declaration.
• memory-model: the reasoning behind the cancel-flag Ordering.
• rust-panic-safety: the panic policy at a C ABI boundary.
• rust-jni and rust-swift-ffi: the raw binding mechanics (thread attachment, callbacks, handle and context lifetime). Apply this skill's error, progress, and cancel protocol on top of them.
• rust-observability: the tracing subscriber that receives redacted events.