transpile: emit VaList::next_arg instead of arg for edition 2024 #2033

rust-lang/rust#155614 renamed VaList::arg to VaList::next_arg, in nightlies from 2026-04-24 on. c2rust still emits .arg::<T>() for va_arg in every edition, so --edition 2024 output for a function that uses va_arg only builds on nightlies before that date.

c2rust-transpile/src/translator/variadic.rs (line 238 on master a9bcc81) emits mk().ident("arg") unconditionally, while mk_va_list_ty and mk_va_list_copy in the same file already switch on the edition. The Edition2024 toolchain is pinned to nightly-2026-03-03 in c2rust-rust-tools/src/lib.rs, which predates the rename, so nothing in the tree notices.

Repro

#include <stdarg.h>

int sum(int n, ...) {
    va_list ap;
    va_start(ap, n);
    int s = 0;
    while (n--) s += va_arg(ap, int);
    va_end(ap);
    return s;
}

c2rust transpile --edition 2024 compile_commands.json --emit-build-files -o out

This emits:

    let mut ap: ::core::ffi::VaList;
    ap = c2rust_args.clone();
    ...
        s += ap.arg::<::core::ffi::c_int>();

cargo check on the output passes with the pinned nightly-2026-03-03. With nightly-2026-05-11 and nightly-2026-08-02 it fails:

error[E0599]: no method named `arg` found for struct `VaList<'a>` in the current scope
  --> src/va.rs:25:17
   |
25 |         s += ap.arg::<::core::ffi::c_int>();
   |                 ^^^ method not found in `VaList<'_>`

Tested with c2rust built from f19c646. The code is unchanged on master.

Suggestion

For Edition2024, emit next_arg and move the pinned toolchain to nightly-2026-04-24 or later. c_variadic was stabilized in rust-lang/rust#155697 (merged 2026-07-22), so with a toolchain from after that, the #![feature(c_variadic)] attribute can go too.

Context: Tractor-Crisp builds c2rust's edition-2021 output on nightly-2026-08-02 and now rewrites .arg::< to .next_arg::< after transpiling (GaloisInc/Tractor-Crisp#211).