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/1303-ffi-objc-block-capture.sx

@@ -0,0 +1,17 @@
+// A capturing closure rides through `xx ... : Block` and the
+// captured state survives across the call. The block's sx_env field
+// holds the closure's env pointer; the invoke trampoline restores it
+// before delegating.
+
+#import "modules/std.sx";
+#import "modules/std/objc_block.sx";
+
+main :: () -> s32 {
+  x : s64 = 42;
+  y : s64 = 100;
+  cl := () => { print("x + y = {}\n", x + y); };
+  b : Block = xx cl;
+  invoke_fn : (*Block) -> void callconv(.c) = xx b.invoke;
+  invoke_fn(@b);
+  0;
+}