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/1200-ffi-callconv-c-callbacks.sx

@@ -0,0 +1,16 @@
+// `callconv(.c)` on function pointers passed to foreign callbacks — ensures
+// the function uses C ABI so it can be safely invoked from `#foreign`
+// functions like SDL_AddEventWatch.
+
+#import "modules/std.sx";
+
+// A function with C calling convention
+add_c :: (a: s64, b: s64) -> s64 callconv(.c) {
+    a + b;
+}
+
+main :: () {
+    // Call it directly — should work like any other function
+    result := add_c(10, 32);
+    print("callconv(.c): {}\n", result);
+}