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/0512-packs-pack-runtime-index.sx

@@ -0,0 +1,29 @@
+// Variadic heterogeneous type packs — Step 2.6: indexing a pack with a
+// RUNTIME index is a compile error.
+//
+// Per locked Decision 1, a pack is comptime-only and has NO runtime
+// representation — so `args[i]` is valid only when `i` is a compile-time
+// constant (a literal, or an `inline for` cursor). A runtime index (here a
+// `while`-loop counter) must produce a clear diagnostic, not the confusing
+// "unresolved 'args'" the slice-index fall-through used to give. To walk a
+// pack, use `inline for 0..args.len: (i) { ... }`, which unrolls so each
+// `args[i]` is a comptime index.
+
+#import "modules/std.sx";
+
+count_anys :: (..$args) -> s64 {
+    total : s64 = 0;
+    i : s64 = 0;
+    while i < args.len {
+        x : Any = args[i];   // ERROR: runtime index into a comptime-only pack
+        _ = x;
+        total = total + 1;
+        i = i + 1;
+    }
+    return total;
+}
+
+main :: () -> s32 {
+    print("{}\n", count_anys(10, "hi", 2.5, true));
+    return 0;
+}