Trailing `args: ..T` on a #foreign declaration now lowers to the C
calling convention's `...` instead of sx-side slice-packing. Drops
the per-arity #foreign-shim workaround for callers of variadic C
APIs (__android_log_print, printf-family, etc.). Closes issue-0043.
- IR: Function.is_variadic on inst.Function; declareFunction drops
the variadic param from the IR signature for foreign+variadic
decls.
- emit_llvm: LLVMFunctionType receives is_var_arg=1 when the flag
is set; call lowering passes extras through unchanged.
- Lowering: packVariadicCallArgs early-outs for foreign+variadic
(no slice-pack); new promoteCVariadicArgs applies C default
argument promotion (bool/s8/s16/u8/u16 -> s32, f32 -> f64) to
extras past the fixed param count.
- Test: examples/ffi-foreign-cvariadic.sx + .c exercise s64/f64/s32
returns through C va_arg over s32/f64/*u8 element types.
134 host + 6 cross tests pass on the WIP-less baseline.
