Rivet 0.4.0

• Require Racket 9.0+ with the CS runtime (base #:version "9.0"); the legacy BC engine and pre-9.0 releases are no longer supported. State the floor in the landing page and getting-started guides.
• Add an agent-native project contract: generated AGENTS.md guidance plus raco rivet inspect --json for versioned project identity, native edit points, target maturity, generated-path ownership, and safe command discovery.
• Add validated application resource declarations, stable Racket-side resource lookup, Windows .ico compilation, and macOS .icns bundle metadata. Resources now participate in build, package, and verification instead of relying on platform-specific copy scripts.
• Promote Linux from a manually built GTK4 experiment to a developer preview: new projects include the Linux host, code generation emits a rivet::linux_runtime client, and doctor, build, dev, package, and verify support a statically embedded Racket CS runtime with real end-to-end CI.
• Make the portable Swift runtime available to iOS/iPadOS 16 and watchOS 9, and add a bounded, versioned RivetDevice request/response layer with a WatchConnectivity adapter for type-safe phone/watch companion communication.
• Add a Kotlin RVT1 codec foundation for Android, pinned Gradle tooling, and CI coverage against the same protocol vectors used by Racket, C++, and Swift.
• Add an Android coroutine client with bounded concurrent calls, Events, State helpers, backend errors, cancellation ordering, lifecycle enforcement, and deterministic Shutdown/stream cleanup.