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/1229-ffi-extern-cvariadic.c

@@ -0,0 +1,20 @@
+#include <stdarg.h>
+
+long long sx_ext_sum_ints(int n, ...) {
+    va_list ap;
+    va_start(ap, n);
+    long long total = 0;
+    for (int i = 0; i < n; i++) total += va_arg(ap, int);
+    va_end(ap);
+    return total;
+}
+
+double sx_ext_avg_doubles(int n, ...) {
+    va_list ap;
+    va_start(ap, n);
+    double total = 0.0;
+    for (int i = 0; i < n; i++) total += va_arg(ap, double);
+    va_end(ap);
+    if (n == 0) return 0.0;
+    return total / n;
+}