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/1226-ffi-export-fn.c

@@ -0,0 +1,8 @@
+#include "1226-ffi-export-fn.h"
+
+// Defined on the sx side via `export` — a plain C-ABI symbol, no sx context.
+extern int sx_square(int n);
+
+int call_sx_square(int n) {
+    return sx_square(n) + 1;
+}