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/1026-errors-failable-main.sx

@@ -0,0 +1,29 @@
+// Failable `-> !` main entry-point wrapper (ERR step E4.2). A pure-failable
+// main that lets an error reach the function boundary exits 1 and prints the
+// unhandled-error header (with the tag name, via the always-linked tag-name
+// table) plus the return trace to stderr — instead of the old behavior of
+// returning the raw tag id as the exit code with no diagnostic. A successful
+// run (no escaping error) exits 0.
+//
+// Note: the header + trace go to stderr. The test runner merges stderr+stdout,
+// so the snapshot shows them interleaved with the `print` (stdout) lines.
+// Frame locations are placeholders until DWARF (ERR E3.0); count + ordering +
+// the tag name are already meaningful. Expected exit code: 1.
+
+#import "modules/std.sx";
+
+ParseErr :: error { Empty, BadDigit };
+
+inner :: (n: s32) -> (s32, !ParseErr) {
+    if n == 0 { raise error.Empty; }       // pushes a frame
+    if n < 0 { raise error.BadDigit; }
+    return n * 2;
+}
+
+main :: () -> !ParseErr {
+    v := try inner(5);                       // succeeds → v = 10
+    print("v = {}\n", v);
+    w := try inner(0);                       // raises Empty → propagates to main
+    print("w = {}\n", w);                    // never reached
+    return;
+}