el-ffi 0.3.8

Host bindings: uniffi-bindgen-react-native (RN), flutter_rust_bridge v2 (Flutter), wasm-bindgen (web) — ADR-001/009.
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# el-ffi — host bindings (React Native, Flutter, Web)

One Rust API surface exported three ways, so mobile and web apps can call the
SDK in their native idiom (ADR-001, ADR-009, ADR-010):

| Surface | Tool | Output |
|---------|------|--------|
| **React Native** | `uniffi-bindgen-react-native` | TypeScript + JSI C++ + Turbo Module |
| **Flutter** | `flutter_rust_bridge` v2 | Dart opaque handle, `Future`/`Stream` |
| **Web / npm** | `wasm-bindgen` | ESM TypeScript package via `wasm-pack` |

No `unsafe` (`#![forbid(unsafe_code)]`). The crate is `cdylib` + `staticlib` +
`lib` so each toolchain can link the form it needs.

## What it provides

- **`EdgeLlm`** — the flat, FFI-friendly facade, annotated for all three
  surfaces at once:
  - `EdgeLlm::local(model_uri)` — local Candle engine, air-gapped (ADR-002/004).
    An empty `model_uri` uses a deterministic toy model for development;
    a path loads a consumer-supplied GGUF.
  - `EdgeLlm::cloud(model, api_key)` — frontier cloud backend (opt-in, ADR-010).
    **Native only** — see the web limitation below. `api_key` must come from the
    platform keystore, never embedded.
  - `ask(prompt) -> Result<String, SdkError>` — blocking chat.
  - `ask_stream(prompt, |token| …)` — closure streaming (Flutter / FRB v2).
  - `ask_stream_cb(prompt, handler)` — `StreamHandler` callback streaming
    (React Native; UniFFI cannot export `impl FnMut`).
  - `reset()`.
- **`SdkError`** — a thin, FFI-safe projection of `el_core::EdgeError`
  (`el-core`'s `Box<str>`/Rust-specific variants are not FFI-safe). Projects to
  the host language's exception type, or a JS exception on wasm.
- **`StreamHandler`** — the React Native streaming callback interface.

## Usage (Rust side)

```rust
use el_ffi::EdgeLlm;

// Empty path → deterministic toy model; exercises the full binding layer.
let sdk = EdgeLlm::local(String::new())?;
let reply = sdk.ask("hello".into())?;
assert!(!reply.is_empty());

// Streaming (Flutter / closure form):
sdk.ask_stream("hi".into(), |fragment| print!("{fragment}"))?;
# Ok::<(), el_ffi::SdkError>(())
```

## Building the bindings

The Rust binding *surfaces* compile on the host; the cross-target builds and
codegen run via the [`Makefile`](../../../Makefile):

```sh
make build-android    # cargo build --target aarch64-linux-android  (shared lib)
make build-ios        # cargo build --target aarch64-apple-ios       (static lib)
make build-wasm       # wasm-pack build → out/web ESM package

make codegen-rn       # React Native JSI bindings (needs build-android)
make codegen-flutter  # flutter_rust_bridge v2 Dart bindings
make bindings         # all three surfaces
```

Prerequisites (rustup targets, NDK linker, `wasm-pack`,
`uniffi-bindgen-react-native`, `flutter_rust_bridge_codegen`) are documented in
the Makefile header.

## Web limitations

On `wasm32` the local path currently uses a dev-stage echo placeholder until
Candle-on-wasm is wired, and the **cloud backend is not available on web**
(ADR-010 amendment): `el-cloud`'s blocking HTTP transport has no wasm
implementation, so `EdgeLlm.cloud` throws an explicit error there instead of
silently degrading. Use a native binding (React Native / Flutter) for cloud
access.

## Status

Implemented and tested (native + `wasm32` compile). As a workspace member, the
host-target Rust surfaces build and test with the rest of the workspace
(`cargo test --workspace`); the Android / iOS / wasm cross-builds and binding
codegen run separately via the Makefile because those toolchains are installed
out-of-band.

---

Part of the [Edge Intelligence](../../../README.md) workspace. Realizes
[ADR-001](../../../docs/adr/ADR-001-adopt-webassembly-as-cross-platform-sdk-runtime.md),
[ADR-009](../../../docs/adr/ADR-009-flutter-rust-bridge-for-dart-bindings.md),
and [ADR-010](../../../docs/adr/ADR-010-unified-llm-provider-trait-with-opt-in-frontier-egress.md).