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/0504-packs-pack-impl-match.sx

@@ -0,0 +1,46 @@
+// Variadic heterogeneous type packs — `..$args` — impl matching
+// lock-in.
+//
+// Step 1d slice for the pack feature (see
+// ~/.claude/plans/lets-see-options-for-merry-dijkstra.md). Pins
+// today's concrete-only impl-matching behaviour: a user-declared
+// `impl Into(Block) for Closure(..$args) -> $R` does NOT match
+// any concrete closure source type. The xx cast site hits the
+// existing "no `Into(Block) for <src>` impl" diagnostic even
+// though the pack impl is reachable.
+//
+// Next commit (1d.B) teaches `tryUserConversion` /
+// `registerParamImpl` to walk a second `param_impl_pack_map`
+// when the concrete-key miss happens. Pack impls bind their
+// `$args` and `$R` to the concrete source closure's tail types
+// and return; monomorphisation proceeds against those bindings.
+//
+// The `Closure(s32, bool) -> bool` shape is not covered by
+// stdlib's hand-rolled impls (only `Closure() -> void` and
+// `Closure(bool) -> void`) and not covered by 96-block-multi-arg's
+// `Closure(s32, *void) -> void` impl, so the pack impl is the
+// only candidate.
+
+#import "modules/std.sx";
+#import "modules/std/objc_block.sx";
+
+impl Into(Block) for Closure(..$args) -> $R {
+    convert :: (self: Closure(..$args) -> $R) -> Block {
+        .{
+            isa = @_NSConcreteStackBlock,
+            flags = 0,
+            reserved = 0,
+            invoke = null,
+            descriptor = xx @__sx_block_descriptor,
+            sx_env = self.env,
+            sx_fn = self.fn_ptr,
+        };
+    }
+}
+
+main :: () -> s32 {
+    cl := (a: s32, b: bool) => true;
+    b : *Block = xx cl;
+    print("pack impl match ok\n");
+    return 0;
+}