test: migrate examples to XXXX-category-name layout + split expected streams

Rename all example tests/companions to the XXXX-category-test-name scheme
(per-category 100-blocks: basic 0010, types 0100, ... errors 1000,
diagnostics 1100, ffi 1200, ffi-objc 1300, ffi-jni 1400, vectors 1500,
platform 1600). Companions and dir/C fixtures move in lockstep with their
parent test; #import/#source/#include paths rewritten to match.

Expected output now lives in examples/expected/ (a sibling dir of the
tests) split into three streams per the new convention:
  <name>.exit / <name>.stdout / <name>.stderr  (+ optional <name>.ir)

run_examples.sh rewritten: scans examples/ and issues/ for an
expected/<name>.exit marker, captures stdout and stderr separately (no
more 2>&1), compares each stream + exit + optional IR snapshot.

Behavior validated unchanged: every renamed test reproduces its prior
merged output + exit (diffs limited to file paths/basenames embedded in
diagnostics + traces, which correctly reflect the new names). Suite:
292 passed, 0 failed. 50-smoke.sx split + issue relocation + docs follow
in subsequent commits.

examples/1218-ffi-foreign-cvariadic.c

@@ -0,0 +1,30 @@
+#include <stdarg.h>
+
+long long sx_ffi_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_ffi_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;
+}
+
+int sx_ffi_count_args(const char *tag, ...) {
+    (void) tag;
+    va_list ap;
+    va_start(ap, tag);
+    int count = 0;
+    while (va_arg(ap, const char *) != 0) count++;
+    va_end(ap);
+    return count;
+}