tests: IR-snapshot harness — diff sx ir output when .ir present

run_examples.sh now supports an optional `tests/expected/<name>.ir`
sibling to `.txt`/`.exit`. When present, the runner also captures
`sx ir <file>` output, normalizes target-/host-specific noise
(module ID, target triple/datalayout, attribute groups, LLVM's
auto-suffixed %temp numbering), and diffs against the snapshot.
`--update` regenerates it alongside the runtime output.

Catches lowering changes that don't affect what the program prints
— exactly the shape Phase 1.5's selector interning will produce
(same runtime output, very different IR).

First snapshot: `ffi-objc-call-03-selector-sharing.ir`. Today the
test emits four `call ptr @sel_registerName(ptr @str.N)` lines for
its four call sites; after 1.5 we expect two static
`@OBJC_SELECTOR_REFERENCES_<sel>` globals + loads at each call
site. The diff between the two snapshots will be the visible
artifact of the optimization.

examples/ffi-objc-call-03-selector-sharing.sx

@@ -9,12 +9,13 @@
 // at module init. Per call site becomes a single load.
 //
 // Runtime behavior is unchanged before vs. after 1.5; the
-// improvement is visible in `sx ir` only. After 1.5 we should
-// see exactly one `sel_registerName` call per unique selector
-// string in the emitted IR — verify with:
-//
-//   ./zig-out/bin/sx ir examples/ffi-objc-call-03-selector-sharing.sx \
-//       | grep -c "call ptr @sel_registerName"
+// improvement is visible in `sx ir` only. The IR snapshot at
+// `tests/expected/ffi-objc-call-03-selector-sharing.ir` locks
+// in today's shape (4 `call ptr @sel_registerName` instructions,
+// one per call site). After 1.5 lands selector interning, the
+// snapshot updates to ≤2 (one per unique selector string) plus
+// a static `@OBJC_SELECTOR_REFERENCES_<sel>` global and loads at
+// the call sites.
 
 #import "modules/std.sx";
 #import "modules/compiler.sx";