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/0408-protocols-optional-protocol.sx

@@ -0,0 +1,34 @@
+// `?Protocol = null` — optional protocol boxes use sentinel-shape
+// (ctx == null is the "none" state), so they cost no extra storage
+// beyond the protocol's standard 2-pointer layout. Method calls on
+// a non-null optional protocol auto-unwrap and dispatch through the
+// vtable / inline fn-ptrs as usual.
+
+#import "modules/std.sx";
+
+GPU :: protocol {
+  ping :: () -> s64;
+}
+
+Impl :: struct {}
+impl GPU for Impl {
+  ping :: (self: *Impl) -> s64 { 42; }
+}
+
+main :: () -> s32 {
+  g : ?GPU = null;
+  if g != null {
+    print("BAD: g not null at start\n");
+  } else {
+    print("g initially null\n");
+  }
+
+  g = xx @Impl.{};
+  if g != null {
+    n := g.ping();
+    print("after assign: g.ping() = {}\n", n);
+  } else {
+    print("BAD: g still null after assign\n");
+  }
+  0;
+}