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/0801-memory-slices.sx

@@ -0,0 +1,31 @@
+#import "modules/std.sx";
+
+main :: () {
+    arr : [5]s32 = .[3, 1, 4, 1, 5];
+    print("arr.len = {}\n", arr.len);
+
+    // subslice array
+    sub := arr[1..4];
+    print("arr[1..4] = {}\n", sub);
+    print("sub.len = {}\n", sub.len);
+
+    // open-ended
+    head := arr[..3];
+    tail := arr[2..];
+    print("arr[..3] = {}\n", head);
+    print("arr[2..] = {}\n", tail);
+
+    // slice of slice
+    sl : []s32 = .[10, 20, 30, 40, 50];
+    mid := sl[1..4];
+    print("sl[1..4] = {}\n", mid);
+    rest := mid[1..];
+    print("mid[1..] = {}\n", rest);
+
+    // string subslicing
+    msg := "hello world";
+    word := msg[6..11];
+    print("msg[6..11] = {}\n", word);
+    prefix := msg[..5];
+    print("msg[..5] = {}\n", prefix);
+}