Extern range

# A ranged extern return is a contract checked at the door (decided 2026-09-22): outside it, the call fails out_of_range.
extern.C fn strlen ['s': ptr[uint8]] -> uint64[min 0, max 10];      # C promises at most 10; checked at the door

MAIN {
    let 'short' := "hello";
    let 'long' := "this is longer than ten";
    no_check let 'a' := strlen[lend 'short'] otherwise 99;
    no_check let 'b' := strlen[lend 'long'] otherwise 99;    # 23 is outside 0..10: fails out_of_range
    let 'c': int8 = 'a';                                    # proven by the bound: at most 10
    std::print.useful['a', " ", 'b', " ", 'c', newln];
    no_check let 'd' := strlen[lend 'long'] otherwise { out_of_range { std::print.useful["C lied", newln]; std::exit[3]; } };
}