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/ffi-05-string-args.c

@@ -0,0 +1,25 @@
+#include "ffi-05-string-args.h"
+
+int ffi_strlen(const char *s) {
+    int n = 0;
+    while (s[n] != 0) n++;
+    return n;
+}
+
+int ffi_first_byte(const char *s) {
+    return (int)(unsigned char)s[0];
+}
+
+int ffi_sum_bytes(const unsigned char *buf, int len) {
+    int total = 0;
+    for (int i = 0; i < len; i++) total += buf[i];
+    return total;
+}
+
+void ffi_write_byte(unsigned char *buf, int idx, unsigned char val) {
+    buf[idx] = val;
+}
+
+const char* ffi_static_greeting(void) {
+    return "hello from C";
+}