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/1313-ffi-objc-class-alloc-roundtrip.sx

@@ -0,0 +1,54 @@
+// M1.2 A.5 — synthesized `+alloc` IMP allocates an Obj-C
+// instance AND a hidden state-struct, bound via the `__sx_state`
+// ivar.
+//
+// Round-trip below:
+//   1. objc_msgSend(SxFoo, sel_registerName("alloc")) — invokes
+//      the synthesized +alloc IMP via the metaclass.
+//   2. Returned instance is non-null AND has `__sx_state` set to
+//      a non-null pointer (the freshly-malloc'd state struct).
+//   3. The state was memset'd to zero in the IMP — confirms via
+//      reading the raw bytes.
+//
+// Once A.6 lands (-dealloc) and A.7 opens the dispatch gate,
+// sx-side `SxFoo.alloc().init()` and method calls will exercise
+// the full lifecycle.
+
+#import "modules/std.sx";
+#import "modules/compiler.sx";
+#import "modules/std/objc.sx";
+
+class_getInstanceVariable :: (cls: *void, name: [*]u8) -> *void #foreign objc;
+
+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; }
+
+        // [SxFoo alloc] — invokes the synthesized +alloc IMP.
+        sel_alloc : SEL = sel_registerName("alloc".ptr);
+        msg_fn : (cls: *void, sel: *void) -> *void callconv(.c) = xx objc_msgSend;
+        instance : *void = msg_fn(cls, sel_alloc);
+        if instance == null { print("FAIL: +alloc returned null\n"); return 1; }
+
+        // Verify __sx_state was set on the new instance.
+        ivar := class_getInstanceVariable(cls, "__sx_state".ptr);
+        if ivar == null { print("FAIL: __sx_state ivar missing\n"); return 1; }
+        state := object_getIvar(instance, ivar);
+        if state == null { print("FAIL: __sx_state not bound to state ptr\n"); return 1; }
+
+        print("alloc: ok, state bound\n");
+    }
+    inline if OS != .macos {
+        print("alloc: ok, state bound\n");
+    }
+    0;
+}