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.c

@@ -0,0 +1,25 @@
+#include "1213-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";
+}