Description

When a func-typed variable is used inside string interpolation ${}, the typechecker does not reject it. The codegen then leaks the raw C function pointer as a decimal integer string into the program output.

Repro

do greet() -> string {
    return "hi"
}

do main() {
    mut f = ()greet
    println("${f}")
}

Expected: Typechecker rejects with E3041 — function references cannot be interpolated.

Actual: Compiles and prints the raw pointer address as a decimal string (e.g., 4334929444).

Root cause

The interpolation validation only checked for bare func type via strcmp(pt->name, "func") == 0. Typed function references like func()->string have TK_FUNCTION kind, which was not being checked.