FFI (Foreign Function Interface) call compilation and runtime behavior.

### Summary

An `--ffi` bound call is silently dropped when its result initializes a local that is never reassigned. The build succeeds with no diagnostic and the program dies at load with `Uncaught ReferenceError: is not defined`.

The trigger is **single assignment**, not the `const` keyword.

### Repro

`native.c`

```c
double sg_f64(double v) { return v * 2.0; }
```

`ffi.json`

```json
{
"ffi_format": 1,
"functions": [
{ "name": "sgF64", "symbol": "sg_f64", "params": ["f64"], "returns": "f64" }
],
"libraries": ["./libnative.a"],
"system_libraries": []
}
```

`main.ts`

```ts
declare function sgF64(v: number): number;

function main(): void {
const viaConst = sgF64(21);
console.log(`const: ${viaConst}`);
}

main();
```

```console
$ clang -O2 -c native.c -o native.o && ar rcs libnative.a native.o
$ scriptc build main.ts --ffi ffi.json -o repro
$ ./repro
Uncaught ReferenceError: sgF64 is not defined
```

Expected `const: 42`. Note nothing prints at all: the failure is at load, not at the statement.

### What works and what doesn't

```ts
let a = sgF64(21); console.log(a); // FAILS
const a = sgF64(21); console.log(a); // FAILS
let a = sgF64(21); a += 1; console.log(a); // works
console.log(sgF64(21)); // works (no binding)
function f(v: number) { return sgF64(v); } // works (no binding)
const o = { v: sgF64(5) }; console.log(o.v); // works
let s = 0; s += sgF64(7); // works
```

Making th...