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/1214-ffi-06-callback.c

@@ -0,0 +1,9 @@
+#include "1214-ffi-06-callback.h"
+
+int ffi_apply_callback(int (*cb)(int), int value) {
+    return cb(value);
+}
+
+int ffi_apply_callback2(int (*cb)(void *ctx, int v), void *ctx, int v) {
+    return cb(ctx, v);
+}