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/1103-diagnostics-err-bad-variant.sx

@@ -0,0 +1,22 @@
+// A match arm with a variant name that doesn't exist on the subject's
+// enum/tagged-union produces `error: no variant 'X' on type 'Y'` instead of
+// falling back to the arm index (which used to cause duplicate switch cases
+// and LLVM verification failures).
+
+#import "modules/std.sx";
+
+Shape :: enum {
+    circle: f32;
+    rect: struct { w, h: f32; };
+    none;
+}
+
+main :: () {
+    s :Shape = .circle(3.14);
+    if s == {
+        case .circle: (r) { print("r={}\n", r); }
+        case .Bogus: (x) { print("bogus={}\n", x); }
+        case .none: print("none\n");
+        case .rect: print("rect\n");
+    }
+}