ffi #foreign: C-variadic tail via args: ..T

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.

tests/expected/ffi-foreign-cvariadic.exit

@@ -0,0 +1 @@
+0

tests/expected/ffi-foreign-cvariadic.txt

@@ -0,0 +1,5 @@
+sum_ints(3, 10, 20, 30) = 60
+sum_ints(0)             = 0
+avg_doubles(2)          = 2.000000
+avg_doubles(3)          = 2.000000
+count_args(3 strs)      = 3