ffi 0.2: fold Pair64 + Quad32 back into small-struct baseline

Now that emit_llvm.zig bridges the struct<->[2 x i64] ABI mismatch
(previous commit), the 9..16-byte integer-only shapes round-trip
cleanly. Extended `examples/ffi-02-small-struct.sx` to cover all
four aggregate ABI slots in one place:

  Vec2   — 8 B,  two f32    (register pair, float)
  Vec4f  — 16 B, four f32   (HFA — homogeneous float aggregate)
  Pair64 — 16 B, two s64    (9..16 B int — [2 x i64] coercion slot)
  Quad32 — 16 B, four s32   (same slot as Pair64)

Vendor helpers (`vendors/ffi_structs/{ffi_structs.h,ffi_structs.c}`)
grow `ffi_pair64_*` + `ffi_quad32_*` companions. Snapshot updated
to capture the full output. 89/89 regression tests pass.

`examples/101-ffi-medium-struct.sx` keeps a minimal focused repro
of the Pair64 case so the issue's emergence-and-fix history stays
greppable.

vendors/ffi_structs/ffi_structs.c

@@ -27,3 +27,31 @@ Vec4f ffi_vec4f_reverse(Vec4f v) {
 float ffi_vec4f_sum(Vec4f v) {
     return v.x + v.y + v.z + v.w;
 }
+
+Pair64 ffi_pair64_make(long long a, long long b) {
+    Pair64 r = { a, b };
+    return r;
+}
+
+Pair64 ffi_pair64_swap(Pair64 p) {
+    Pair64 r = { p.b, p.a };
+    return r;
+}
+
+long long ffi_pair64_sum(Pair64 p) {
+    return p.a + p.b;
+}
+
+Quad32 ffi_quad32_make(int a, int b, int c, int d) {
+    Quad32 r = { a, b, c, d };
+    return r;
+}
+
+Quad32 ffi_quad32_reverse(Quad32 q) {
+    Quad32 r = { q.d, q.c, q.b, q.a };
+    return r;
+}
+
+int ffi_quad32_sum(Quad32 q) {
+    return q.a + q.b + q.c + q.d;
+}