Add rust_backend/README.md: crate map and dependencies, vendored patches, toolchain, host and Android/iOS build commands (all pass --locked; none pass --offline), NDK version and ABIs, UniFFI binding outputs and consumers, and the test layout.
Derived from rust_backend/Cargo.toml, rust-toolchain.toml, crate Cargo.toml files, uniffi.toml, the scripts/ build and check scripts, android/app/build.gradle.kts, ios/Podfile and .github/workflows/ci.yml.
