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/0519-packs-pack-bare-value.sx

@@ -0,0 +1,35 @@
+// Variadic heterogeneous type packs — step 4 final slice (4A
+// bare): `$args` referenced bare (without `[...]` indexing) in
+// expression position should evaluate to a comptime `[]Type`
+// slice value — the whole pack passed through as data so
+// builder fns can walk it.
+//
+// Use case (eventual): step 5's generic Into(Block) impl body
+//   convert :: (self: Closure(..$args) -> $R) -> Block {
+//       #insert build_block_convert($args, $R);
+//   }
+// where `build_block_convert(args: []Type, ret: Type) -> string`
+// is a regular sx fn the interp executes — it walks `args` to
+// emit a trampoline fn matching the per-mono signature.
+//
+// Today the parser-arm I wrote for `$<ident>[<int>]` (commit
+// fd03b58, M5.A.next.4.3) REQUIRES the `[` after the pack
+// name; bare `$args` hits a focused diagnostic. This file
+// pins that rejection. Next commit makes the `[` optional —
+// no `[` yields a `comptime_pack_ref` AST node which lowering
+// converts to a `[N x Type]` aggregate of `const_type` values.
+
+#import "modules/std.sx";
+
+len_of :: (..$args) -> s64 {
+    list := $args;
+    return list.len;
+}
+
+main :: () -> s32 {
+    print("{}\n", len_of());
+    print("{}\n", len_of(42));
+    print("{}\n", len_of(1, 2, 3));
+    print("{}\n", len_of("a", true, 3.14, "b"));
+    return 0;
+}