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/expected/0900-optionals-optionals.stdout

@@ -0,0 +1,20 @@
+x = 42
+y = null
+x! = 42
+x ?? 0 = 42
+y ?? 99 = 99
+if-bind x: 42
+if-bind y: none
+match some: 42
+match none: 0
+p?.value = 10
+q?.value = 0
+o1.inner?.val = 99
+o2.inner?.val = 0
+narrowed a: 10
+guard 42: 42
+guard null: 0
+both: 10 20
+guard2: 7
+default next: null
+comptime: 141