ffi: move test-companion .c/.h next to their .sx (drop vendors/ namespace)

vendors/ is a third-party namespace (stb_image, kb_text_shape, etc.);
test fixtures don't belong there. The .c/.h companion files for the
Phase-0 FFI baselines now sit alongside the .sx that drives them in
examples/, with matching basenames:

  examples/ffi-01-primitives.{sx,c,h}    <- was vendors/ffi_primitives/
  examples/ffi-02-small-struct.{sx,c,h}  <- was vendors/ffi_structs/
  examples/ffi-03-large-struct.{sx,c,h}  <- was vendors/ffi_large_struct/
  examples/ffi-04-fp-struct.{sx,c,h}     <- was vendors/ffi_fp_struct/
  examples/ffi-05-string-args.{sx,c,h}   <- was vendors/ffi_strings/
  examples/ffi-06-callback.{sx,c,h}      <- was vendors/ffi_callback/
  examples/101-ffi-medium-struct.{sx,c}  <- was vendors/ffi_medium_struct/

`#source` / `#include` paths in the .sx files become bare filenames
(no prefix) since imports.zig's base_dir resolution finds them
relative to the importing .sx file's directory.

`library/vendors/sx_ffi_resolve_test/` stays put — that one's the
whole point: regression coverage for the stdlib-search branch of
the resolution chain, so it must live where ONLY that branch can
find it.

94/94 regression tests pass.

examples/101-ffi-medium-struct.c

@@ -0,0 +1,13 @@
+// Companion to examples/101-ffi-medium-struct.sx — a single
+// roundtrip through a 16-byte integer-only struct. Pinned in a
+// dedicated example because integer aggregates in this size class
+// route through emit_llvm.zig's `[2 x i64]` ABI coercion slot, a
+// different path from the small (≤8 B) integer struct, the 16-byte
+// HFA, and the >16 B byval-pointer cases.
+
+typedef struct { long long a; long long b; } Pair64;
+
+Pair64 ffi_pair64_swap(Pair64 p) {
+    Pair64 r = { p.b, p.a };
+    return r;
+}