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/0018-basic-quicksort.sx

@@ -0,0 +1,35 @@
+#import "modules/std.sx";
+
+quick_sort :: (items: []$T) {
+    partition :: (items: []T, lo: s64, hi: s64) -> s64 {
+        pivot := items[hi];
+        i := lo - 1;
+        j := lo;
+        while j < hi {
+            if items[j] < pivot {
+                i += 1;
+                items[i], items[j] = items[j], items[i];
+            }
+            j += 1;
+        }
+        i += 1;
+        items[i], items[hi] = items[hi], items[i];
+        i;
+    }
+
+    sort :: (items: []T, lo: s64, hi: s64) {
+        if lo < hi {
+            pi := partition(items, lo, hi);
+            sort(items, lo, pi - 1);
+            sort(items, pi + 1, hi);
+        }
+    }
+
+    sort(items, 0, items.len - 1);
+}
+
+main :: () {
+    arr : []s64 = .[333, 2, 3, 5, 2, 2, 3, 4, 5, 6, 6, 1];
+    quick_sort(arr);
+    print("{}\n", arr);
+}