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/1312-ffi-objc-class-method-dispatch.sx

@@ -0,0 +1,51 @@
+// M1.2 A.4b.iii — instance-method dispatch through the Obj-C
+// runtime. Each instance method now gets a C-ABI IMP trampoline
+// registered via 'class_addMethod' at class-pair init time. The
+// runtime can dispatch 'objc_msgSend(obj, sel)' to the
+// trampoline, which reads the '__sx_state' ivar to find the
+// state struct and forwards to the sx body.
+//
+// End-to-end (verifies registration only — sx-side
+// 'obj.bump()' calls still bail at the M1.2 A.7 dispatch gate
+// until +alloc/-dealloc (A.5/A.6) land too):
+//   1. class_getMethodImplementation(SxFoo, sel_registerName("bump"))
+//      returns a non-null IMP — proves the trampoline is wired.
+
+#import "modules/std.sx";
+#import "modules/compiler.sx";
+#import "modules/std/objc.sx";
+
+class_getMethodImplementation :: (cls: *void, sel: *void) -> *void #foreign objc;
+
+SxFoo :: #objc_class("SxFoo") {
+    counter: s32;
+
+    bump :: (self: *Self) {
+        self.counter += 1;
+    }
+
+    add :: (self: *Self, n: s32) {
+        self.counter += n;
+    }
+}
+
+main :: () -> s32 {
+    inline if OS == .macos {
+        cls : Class = objc_getClass("SxFoo".ptr);
+        if cls == null { print("FAIL: SxFoo not registered\n"); return 1; }
+
+        sel_bump : SEL = sel_registerName("bump".ptr);
+        imp_bump : *void = class_getMethodImplementation(cls, sel_bump);
+        if imp_bump == null { print("FAIL: bump IMP missing\n"); return 1; }
+
+        sel_add : SEL = sel_registerName("add:".ptr);
+        imp_add : *void = class_getMethodImplementation(cls, sel_add);
+        if imp_add == null { print("FAIL: add: IMP missing\n"); return 1; }
+
+        print("IMP: bump ok, add: ok\n");
+    }
+    inline if OS != .macos {
+        print("IMP: bump ok, add: ok\n");
+    }
+    0;
+}