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/0301-closures-fn-pointers.sx

@@ -0,0 +1,28 @@
+#import "modules/std.sx";
+
+add :: (a: s32, b: s32) -> s32 { a + b; }
+mul :: (a: s32, b: s32) -> s32 { a * b; }
+
+apply :: (f: (s32, s32) -> s32, x: s32, y: s32) -> s32 {
+    f(x, y);
+}
+
+main :: () {
+    // Store function in variable
+    fp : (s32, s32) -> s32 = add;
+    print("fp(3,4) = {}\n", fp(3, 4));
+
+    // Reassign to different function
+    fp = mul;
+    print("fp(3,4) = {}\n", fp(3, 4));
+
+    // Pass function pointer as argument
+    print("apply(add,5,6) = {}\n", apply(add, 5, 6));
+    print("apply(mul,5,6) = {}\n", apply(mul, 5, 6));
+}
+
+// ** stdout **
+//fp(3,4) = 7
+//fp(3,4) = 12
+//apply(add,5,6) = 11
+//apply(mul,5,6) = 30