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/0902-optionals-optional-all-null.sx

@@ -0,0 +1,39 @@
+// Struct with multiple `?f32` fields, all set to `null` simultaneously, passed
+// into a protocol-dispatched method. Exercises the all-null-payload path through
+// the boxed call.
+
+#import "modules/std.sx";
+
+ProposedSize :: struct {
+    width: ?f32;
+    height: ?f32;
+}
+
+Sizable :: protocol {
+    size :: (proposal: ProposedSize) -> f32;
+}
+
+Widget :: struct {}
+
+impl Sizable for Widget {
+    size :: (self: *Widget, proposal: ProposedSize) -> f32 {
+        w := if pw := proposal.width { pw; } else { 100.0; };
+        h := if ph := proposal.height { ph; } else { 100.0; };
+        w + h;
+    }
+}
+
+main :: () -> void {
+    w := Widget.{};
+    s : Sizable = w;
+
+    // These work:
+    r1 := s.size(ProposedSize.{ width = 50.0, height = null });
+    print("r1 = {}\n", r1);
+    r2 := s.size(ProposedSize.{ width = null, height = 50.0 });
+    print("r2 = {}\n", r2);
+
+    // This fails with "scalar-to-vector conversion failed":
+    r3 := s.size(ProposedSize.{ width = null, height = null });
+    print("r3 = {}\n", r3);
+}