test: group examples into per-category folders

Move examples/*.sx and their expected/ snapshots into per-category
subfolders (examples/<category>/...). Folder = leading filename token,
with ffi-objc/ffi-jni kept whole; filenames are unchanged. The corpus
runner and LSP sweep now discover each category's expected/ dir, while
issues/ stays flat. Example 1058's repo-root-relative companion import
is made file-relative. Path strings embedded in 164 snapshots were
regenerated (path-only changes). Test-layout docs in CLAUDE.md updated.

examples/ffi/1213-ffi-05-string-args.h

@@ -0,0 +1,14 @@
+// String / byte-pointer FFI baselines. Covers the three shapes
+// callers actually use at the sx ↔ C boundary:
+//   - null-terminated `[:0]u8`              (C-style string)
+//   - raw byte pointer `[*]u8` + length     (slice-style)
+//   - sx `string` decayed to `ptr`          (the slice-decay branch
+//                                            in coerceArg pulls .ptr)
+
+#include <stddef.h>
+
+int    ffi_strlen     (const char *s);
+int    ffi_first_byte (const char *s);
+int    ffi_sum_bytes  (const unsigned char *buf, int len);
+void   ffi_write_byte (unsigned char *buf, int idx, unsigned char val);
+const char* ffi_static_greeting(void);