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/1019-errors-failable-discard-reject.sx

@@ -0,0 +1,30 @@
+// Failable error-slot discard rejection (ERR step E1.8 — discard slice). The
+// error slot of a value-carrying failable cannot be dropped on a bare
+// destructure: it must be bound (`v, err := …`) and handled, or the failure
+// routed through `try` / `catch` / `or value` (all of which strip the error
+// channel, so they don't reach this check). Two rejected shapes here:
+//   (1) omitting the error slot entirely (fewer names than slots), and
+//   (2) binding it to `_`.
+// This file is expected to FAIL compilation (exit 1).
+//
+// Run: ./zig-out/bin/sx run examples/236-failable-discard-reject.sx
+
+#import "modules/std.sx";
+
+E :: error { Bad, Empty }
+
+pair :: (n: s32) -> (s32, s32, !E) {
+    if n < 0 { raise error.Bad; }
+    return (n, n + 1);
+}
+
+parse :: (n: s32) -> (s32, !E) {
+    if n < 0 { raise error.Bad; }
+    return n * 2;
+}
+
+main :: () -> s32 {
+    a, b := pair(5);     // ERROR: error slot omitted (3 slots, 2 names)
+    v, _ := parse(5);    // ERROR: error slot discarded with `_`
+    return a + b + v;
+}