feat: add foreign function support

PLAN.md

@@ -49,9 +49,9 @@ A Rust-flavored, C-targeting language - built pipeline-first.
 
 To successfully write a simple ray tracer, we need continuous math, data structures, and I/O. The following path establishes these prerequisites:
 
-- [ ] **Floating-Point Support:** Add `f32`/`f64` types, decimal literals, and Cranelift lowering for `fadd`, `fmul`, etc.
-- [ ] **FFI & Interop:** Implement `extern fn` declarations to bind C standard library functions (like `putchar` or `printf`) for `.ppm` image output.
-- [ ] **Type Casting:** Add the `as` operator to convert floating-point color bounds `[0.0, 1.0]` into integer byte formats `[0, 255]`.
+- [x] **Floating-Point Support:** Add `f32`/`f64` types, decimal literals, and Cranelift lowering for `fadd`, `fmul`, etc.
+- [x] **FFI & Interop:** Implement `extern fn` declarations to bind C standard library functions (like `putchar` or `printf`) for `.ppm` image output.
+- [x] **Type Casting:** Add the `as` operator to convert floating-point color bounds `[0.0, 1.0]` into integer byte formats `[0, 255]`.
 - [ ] **Pointers:** Add pointer types (`*T`), address-of (`&`), and dereference (`*`) operators.
 - [ ] **Structs:** Add `struct` definitions, initializers, and field access (`ray.origin.x`) to represent 3D vectors, rays, and spheres.
 - [ ] **Arrays:** Add fixed-size arrays (`[T; N]`) or heap allocations for the scene and framebuffers.