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/1315-ffi-objc-self-class-accessor.sx

@@ -0,0 +1,48 @@
+// M1.3 — `obj.class` accessor on Obj-C pointers.
+//
+// Any Obj-C-class pointer (foreign or sx-defined) can be probed
+// for its runtime class object via `obj.class`. Lowers to
+// `object_getClass(obj)`. Returns `Class` (alias for *void —
+// parameterized `Class(T)` covariance is M1.1.b).
+//
+// Verifies both shapes:
+//   1. (*SxFoo).class — sx-defined class. Returns the SxFoo Class.
+//   2. (*NSObject).class — foreign class via stdlib. Returns NSObject's
+//      Class.
+
+#import "modules/std.sx";
+#import "modules/compiler.sx";
+#import "modules/std/objc.sx";
+
+NSObjectFwd :: #foreign #objc_class("NSObject") {
+    alloc :: () -> *NSObjectFwd;
+    init  :: (self: *NSObjectFwd) -> *NSObjectFwd;
+}
+
+SxFoo :: #objc_class("SxFoo") {
+    counter: s32;
+    alloc :: () -> *SxFoo;
+    bump :: (self: *Self) { self.counter += 1; }
+}
+
+main :: () -> s32 {
+    inline if OS == .macos {
+        // sx-defined class round-trip.
+        f := SxFoo.alloc();
+        cls_f : Class = f.class;
+        expected_f : Class = objc_getClass("SxFoo".ptr);
+        if cls_f != expected_f { print("FAIL: SxFoo.class mismatch\n"); return 1; }
+
+        // foreign class round-trip.
+        nso := NSObjectFwd.alloc().init();
+        cls_n : Class = nso.class;
+        expected_n : Class = objc_getClass("NSObject".ptr);
+        if cls_n != expected_n { print("FAIL: NSObject.class mismatch\n"); return 1; }
+
+        print("class accessor: ok\n");
+    }
+    inline if OS != .macos {
+        print("class accessor: ok\n");
+    }
+    0;
+}