ffi 0.2: small struct baseline (Vec2, Vec4f) by-value through #foreign

88/88 regression tests pass (+ffi-02-small-struct).

vendors/ffi_structs/ defines:
  Vec2  — 8 B, two f32 — register-pair (float) ABI
  Vec4f — 16 B, four f32 — homogeneous float aggregate (HFA) on AAPCS64

Both pass cleanly today: the sx-side struct declarations match the C
ABI for these float-only shapes, and the call-site / foreign-decl
type representations agree.

`#source` only (no `#include`) — c_import's type mapping rewrites
struct-typed params/returns to *void, which would link but pass
through the wrong ABI silently. The hand-written #foreign decls keep
sx's struct types end to end.

16-byte integer-only shapes (`{s64, s64}`, `{s32, s32, s32, s32}`)
discovered to trip the LLVM verifier (`[2 x i64]` vs `{ i64, i64 }`
mismatch between foreign decl and call site). Excluded from this
baseline; filed separately in the next commit as issue-0036.

vendors/ffi_structs/ffi_structs.c

@@ -0,0 +1,29 @@
+#include "ffi_structs.h"
+
+Vec2 ffi_vec2_make(float x, float y) {
+    Vec2 r = { x, y };
+    return r;
+}
+
+Vec2 ffi_vec2_swap(Vec2 v) {
+    Vec2 r = { v.y, v.x };
+    return r;
+}
+
+float ffi_vec2_sum(Vec2 v) {
+    return v.x + v.y;
+}
+
+Vec4f ffi_vec4f_make(float x, float y, float z, float w) {
+    Vec4f r = { x, y, z, w };
+    return r;
+}
+
+Vec4f ffi_vec4f_reverse(Vec4f v) {
+    Vec4f r = { v.w, v.z, v.y, v.x };
+    return r;
+}
+
+float ffi_vec4f_sum(Vec4f v) {
+    return v.x + v.y + v.z + v.w;
+}

vendors/ffi_structs/ffi_structs.h

@@ -0,0 +1,18 @@
+// FFI struct-marshalling baselines. Two shapes covered today:
+//   Vec2  — 8 bytes  (two f32)         — register pair, float path
+//   Vec4f — 16 bytes (four f32)        — homogeneous float aggregate (HFA)
+// Declared here so the .c has a header to include; the sx side
+// imports via `#source` only and re-declares the structs natively
+// (c_import currently rewrites struct-typed params/returns to *void,
+// which loses the by-value ABI).
+
+typedef struct { float x; float y; } Vec2;
+typedef struct { float x; float y; float z; float w; } Vec4f;
+
+Vec2  ffi_vec2_make  (float x, float y);
+Vec2  ffi_vec2_swap  (Vec2 v);
+float ffi_vec2_sum   (Vec2 v);
+
+Vec4f ffi_vec4f_make   (float x, float y, float z, float w);
+Vec4f ffi_vec4f_reverse(Vec4f v);
+float ffi_vec4f_sum    (Vec4f v);