Bug: Invalid `TODO` emitted in WAT for FFI variables (`array.new_fixed`)

Bug: Invalid TODO emitted in WAT for FFI variables (array.new_fixed)

Description

The compiler is emitting invalid WebAssembly text (.wat) when encountering
FFI variables. Specifically, the Reference function classifies FFI variables
as 'ffi, but the corresponding code generation branch is unimplemented and
returns the literal symbol 'TODO.

This results in invalid WAT like:

(array.new_fixed $Args 2 (global.get $symbol:quoted-symbol198) TODO)

which causes a compilation failure:

error: expected an instruction
    --> new-term-w-socket-io.wat:95399:74
95399 |     (array.new_fixed $Args 2 (global.get $symbol:quoted-symbol198) TODO)
       |                                                                          ^
* Compilation Error *

Where it comes from

In compiler.rkt, inside Reference:

(case (classify v)
  ...
  [(ffi)        'TODO]
  ...)

The classification is triggered by:

(define (ffi-variable? v)
  (set-member? ffi-primitives-set
               (if (symbol? v) v (syntax-e (variable-id v)))))

and:

[(ffi-variable? v) 'ffi]

So whenever an FFI primitive is referenced, it ends up generating 'TODO
in the output WAT.

Impact

• Produces invalid .wat output
• Breaks wat->wasm compilation
• Blocks use of FFI primitives in compiled programs

Steps to reproduce

Compile a program that references an FFI primitive (e.g. in
new-term-w-socket-io.rkt). The generated .wat will contain TODO
as an operand in an instruction.

Expected behavior

The compiler should emit a valid WebAssembly instruction for FFI variables,
similar to how primitives or globals are handled.

Actual behavior

The compiler emits the literal token TODO, which is not valid WAT syntax.

Suggested fix

Implement the [(ffi) ...] branch in Reference so that FFI variables are
lowered to valid WebAssembly (likely as globals or imported functions),
instead of returning 'TODO.

Start in compiler.rkt at Reference and trace classify, ffi-variable?, and the (ffi) branch. Reproduce with a program such as new-term-w-socket-io.rkt, then determine how FFI variables should be lowered. Done means the generated WAT contains no TODO operand and wat->wasm compiles it successfully.