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/0602-comptime-interp-cast-ptr-cmp.sx

@@ -0,0 +1,32 @@
+// `cast(T) val` inside a conditional, evaluated by the IR interpreter
+// during a post-link callback. The `cast` syntax lowers the type arg
+// (`s64`) as a `placeholder` IR op; the interpreter treats placeholders
+// as undef so the comparison runs through unchanged.
+
+#import "modules/std.sx";
+#import "modules/compiler.sx";
+
+libc :: #library "c";
+popen :: (cmd: [:0]u8, mode: [:0]u8) -> *void #foreign libc;
+puts  :: (s: [:0]u8) -> s32 #foreign libc;
+
+R :: struct { x: s32; }
+
+bug :: (cmd: [:0]u8) -> ?R {
+    f := popen(cmd, "r");
+    if cast(s64) f == 0 { return null; }
+    R.{ x = 1 };
+}
+
+post_link :: () -> bool {
+    if r := bug("echo hi") { puts("ok"); } else { puts("null"); }
+    true;
+}
+
+configure :: () {
+    opts := build_options();
+    opts.set_post_link_callback(post_link);
+}
+#run configure();
+
+main :: () { print("rt\n"); }