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/1310-ffi-objc-class-registration.sx

@@ -0,0 +1,43 @@
+// M1.2 A.4 — class-pair registration with the Obj-C runtime.
+//
+// Every sx-defined '#objc_class' produces a module-init constructor
+// (registered in '@llvm.global_ctors' AND injected at the top of
+// 'main' for the ORC JIT path) that calls:
+//
+//   super = objc_getClass("NSObject")
+//   cls   = objc_allocateClassPair(super, "SxFoo", 0)
+//   objc_registerClassPair(cls)
+//
+// After the constructor runs, 'objc_getClass("SxFoo")' returns the
+// freshly registered class — the round-trip we verify below.
+//
+// Methods, the '__sx_state' ivar, and the '+alloc' / '-dealloc'
+// overrides land in A.4b / A.5 / A.6; this slice just makes the
+// class EXIST in the runtime.
+
+#import "modules/std.sx";
+#import "modules/compiler.sx";
+#import "modules/std/objc.sx";
+
+SxFoo :: #objc_class("SxFoo") {
+    counter: s32;
+
+    bump :: (self: *Self) {
+        self.counter += 1;
+    }
+}
+
+main :: () -> s32 {
+    inline if OS == .macos {
+        cls : Class = objc_getClass("SxFoo".ptr);
+        if cls == null {
+            print("FAIL: SxFoo not registered\n");
+            return 1;
+        }
+        print("registered: SxFoo\n");
+    }
+    inline if OS != .macos {
+        print("registered: SxFoo\n");
+    }
+    0;
+}