ffi-error-progress-cancellisted

# FFI error, progress, and cancellation ## Purpose Own the end-to-end contract that crosses the FFI boundary for every long-running operation: a stable error taxonomy, a progress event stream, and cooperative cancellation. These three concerns are one contract. If you design them separately, you get an error enum that cannot express "the user pressed Cancel", a progress stream that leaks a worker thread when the consumer goes away, and a cancel call that blocks the UI thread. This skill covers the boundary UX. Related skills cover the parts underneath: - `uniffi-boundary` - binding generation mechanics and UDL/proc-macro shape. - `memory-model` - the exact atomic `Ordering` for the cancel flag and the progress counter. - `rust-panic-safety` and `rust-discipline` - panic policy at the boundary. - `rust-debugging` - panic propagation through generated bindings. - `rust-observability` - the `tracing` subscriber that receives redacted events. - `rust-jni` - the same three protocols over a hand-written JNI boundary. ## When to use - You design or review an FFI method that takes measurable time: archive import, index build, preview generation, large or vector export, batch export, asset import. - You map a core error enum to the flat boundary enum, then on to Kotlin exceptions and Swift `Error`. - You bridge a Rust progress callback into a Kotlin `Flow` or a Swift `AsyncThrowingStream`. - You wire coroutine cancellation or `Task` cancellation down to a Rust job ca