ffi 0.3: large struct baseline (Big24, Big48) through sret return path

90/90 regression tests pass (+ffi-03-large-struct).

vendors/ffi_large_struct/{.h,.c} defines:
  Big24 — 24 B, three s64    (byval params + sret return)
  Big48 — 48 B, six s64      (same path, larger)

`make / rotate-or-reverse / sum` helpers per shape. The sx-side
example imports via `#source` only and declares matching structs +
hand-written #foreign decls.

Snapshot pins today's >16-byte aggregate ABI now that the
emit_llvm.zig sret-return transform is in place (previous commit).
That gives us a regression net for all four C-ABI aggregate slots
in one place:

  ≤8 B int       — i64 coercion          (ffi-01 vec-likes)
  9..16 B int    — [2 x i64] coercion    (ffi-02 Pair64/Quad32, 101)
  16 B HFA       — struct, no coercion    (ffi-02 Vec2/Vec4f)
  >16 B          — byval params + sret    (this commit)

tests/expected/ffi-03-large-struct.exit

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