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.

src/ir/inst.zig

@@ -478,6 +478,13 @@ pub const Function = struct {
     linkage: Linkage = .internal,
     call_conv: CallingConvention = .default,
     source_file: ?[]const u8 = null,
+    /// Variadic tail at the IR signature level. Only `#foreign` decls reach
+    /// IR with this set — sx-side `..T` params are slice-packed before
+    /// lowering, so anything that survives is the C calling convention's
+    /// `...`. emit_llvm passes `is_var_arg=1` to `LLVMFunctionType`; call
+    /// sites apply the standard default argument promotions (s8/s16/bool →
+    /// s32, f32 → f64) to extras past the fixed param count.
+    is_variadic: bool = false,
 
     pub const Param = struct {
         name: StringId,