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/1500-vectors-vector-math.sx

@@ -0,0 +1,44 @@
+#import "modules/std.sx";
+math :: #import "modules/math";
+
+dot :: (a: Vector(3,f32), b: Vector(3,f32)) -> f32 {
+    a.x*b.x + a.y*b.y + a.z*b.z;
+}
+
+cross :: (a: Vector(3,f32), b: Vector(3,f32)) -> Vector(3,f32) {
+    .[a.y*b.z - a.z*b.y, a.z*b.x - a.x*b.z, a.x*b.y - a.y*b.x];
+}
+
+length :: (v: Vector(3,f32)) -> f32 {
+    math.sqrt(dot(v, v));
+}
+
+normalize :: (v: Vector(3,f32)) -> Vector(3,f32) {
+    v / length(v);
+}
+
+vec3 :: (x:f32, y:f32, z:f32) -> Vector(3,f32) {
+    .[x, y, z];
+}
+
+main :: () {
+    a := vec3(1, 0, 0);
+    b := vec3(0, 1, 0);
+
+    // dot product
+    d := dot(a, b);
+    print("dot: {}\n", d);
+
+    // cross product
+    cr := cross(a, b);
+    print("cross: {}\n", cr);
+
+    // length
+    v := vec3(3, 4, 0);
+    len := length(v);
+    print("length: {}\n", len);
+
+    // normalize
+    n := normalize(v);
+    print("norm: {}\n", n);
+}