Add a standalone WebAssembly host interoperability example

## Why

`examples/wasm_module.wave` exercises many WebAssembly backend features at once and is used by CI smoke tests. It includes `extern(c)` and `export(c)` host boundaries, but there is no small self-contained example showing how a JavaScript host actually provides and calls those functions.

## Scope

Add a small WebAssembly interoperability example under `examples/` with a Wave module and a minimal JavaScript host, preferably using Node.js and no third-party packages.

The example should demonstrate:

- importing one host function through `extern(c, "...")`;
- exporting one Wave function through `export(c, "...")`;
- instantiating the generated `.wasm` file from JavaScript;
- providing the required `env` import object;
- calling the Wave export and checking a deterministic result.

Keep this separate from the existing WebAssembly smoke module rather than expanding that file further. Missing-host-import diagnostics remain tracked separately by #453.

## Completion criteria

- [ ] The Wave source passes `wavec check` and builds for `wasm32-unknown-unknown`.
- [ ] The JavaScript host runs with the Node.js version already used by CI and requires no npm dependencies.
- [ ] The host supplies the imported function and successfully calls the exported Wave function.
- [ ] A short README or examples index entry includes the exact build and run commands.
- [ ] Existing `wasm_module.wave`, `wasm_run.wave`, and WASI smoke coverage remain unchanged.

Start by reading examples/wasm_module.wave and examples/wasm_run.wave, then inspect the existing CI Node.js version and WebAssembly build commands. Add a separate Wave source, JavaScript host, and short README or examples index entry; done means wavec check and wasm32-unknown-unknown build pass, the host supplies env and gets the deterministic export result, and the existing smoke files remain unchanged.

Written by the indexing model from the issue text.