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/0023-basic-for-by-ref-capture.sx

@@ -0,0 +1,25 @@
+// `for xs: (*x)` binds each element by pointer — no per-element copy.
+// Mutations write back, and a pointer subject matches through the deref.
+#import "modules/std.sx";
+
+Shape :: enum {
+    circle: f32;
+    none;
+}
+
+main :: () -> s32 {
+    // By-ref mutation writes back into the array (impossible with a value copy).
+    xs : [3]s64 = .[1, 2, 3];
+    for xs: (*x) { x.* = x + 100; }
+    print("{} {} {}\n", xs[0], xs[1], xs[2]);
+
+    // Pointer subject matches through the deref; payload reads through the ref.
+    shapes : [2]Shape = .[.circle(2.0), .none];
+    for shapes: (*s) {
+        if s == {
+            case .circle: (r) { print("circle {}\n", r); }
+            case .none:        { print("none\n"); }
+        }
+    }
+    0;
+}