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/0015-basic-demo.sx

@@ -0,0 +1,15 @@
+std :: #import "modules/std.sx";
+
+vec3 :: (x:f32, y:f32, z:f32) -> std.Vector(3, f32) {
+    .[x,y,z];
+}
+
+main :: () {
+    v1 := vec3(1,0,0);
+    v2 := vec3(0,0,1);
+    s := 0.5;
+
+    sum := (v1 - v2);// math.cross(v1, v2);
+
+    std.print("{}\n", sum);
+}