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/0021-basic-expression-bodied-fn.sx

@@ -0,0 +1,36 @@
+// M1.0 — expression-bodied function declarations.
+//
+// sx's `=>` body form (already used for lambdas) extends to
+// top-level and struct-member function declarations:
+//
+//     name :: (params) -> RetType => expr;
+//
+// Pins three positions: module-top, struct method, niladic.
+
+#import "modules/std.sx";
+
+double :: (x: s32) -> s32 => x * 2;
+
+sum :: (a: s32, b: s32) -> s32 => a + b;
+
+answer :: () -> s32 => 42;
+
+Point :: struct {
+    x: s32;
+    y: s32;
+
+    total :: (self: *Point) -> s32 => self.x + self.y;
+    scaled :: (self: *Point, by: s32) -> s32 => (self.x + self.y) * by;
+}
+
+main :: () -> s32 {
+    print("double: {}\n", double(7));
+    print("sum: {}\n", sum(3, 4));
+    print("answer: {}\n", answer());
+
+    p := Point.{ x = 10, y = 20 };
+    print("total: {}\n", p.total());
+    print("scaled: {}\n", p.scaled(3));
+
+    0;
+}