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/0012-basic-shadow.sx

@@ -0,0 +1,20 @@
+#import "modules/std.sx";
+main :: () -> s32 {
+    x := 42;
+    {
+        print("scope opened\n");
+        defer print("scope closed\n");
+
+        // define a inner variable x shadowing the one define in the outer scope(s)
+        x:= 6;
+        print("scoped x: {}\n", x); //expect 6
+    }
+    print("main x: {}\n", x); //expect 42
+}
+
+// ** stdout **
+// scope opened
+// scoped x: 6
+// scope closed
+// main x: 42
+//