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/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";
+}