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.

tests/run_examples.sh

@@ -25,6 +25,21 @@ normalize() {
     sed 's/0x[0-9a-f]\{4,\}/0xADDR/g'
 }
 
+# Normalize `sx ir` output for snapshot diffing. Strips host-specific
+# noise so a snapshot taken on one macOS machine matches another:
+#   - target triple / datalayout / module-id headers
+#   - function-attribute groups (target-cpu, frame-pointer string)
+#   - LLVM's auto-suffixed temporary names (%add1, %icmp29 → %add, %icmp)
+normalize_ir() {
+    sed -E \
+        -e '/^; ModuleID =/d' \
+        -e '/^source_filename =/d' \
+        -e '/^target datalayout =/d' \
+        -e '/^target triple =/d' \
+        -e '/^attributes #[0-9]+ = \{/d' \
+        -e 's/%([a-z]+)[0-9]+/%\1N/g'
+}
+
 for expected_file in "$EXPECTED_DIR"/*.txt; do
     name=$(basename "$expected_file" .txt)
     sx_file="$ROOT_DIR/examples/${name}.sx"
@@ -45,9 +60,27 @@ for expected_file in "$EXPECTED_DIR"/*.txt; do
         continue
     fi
 
+    # Optional IR-shape snapshot. When tests/expected/<name>.ir exists,
+    # also diff `sx ir <file>` against it. Used to lock down lowering
+    # changes that don't show up in runtime output (e.g., selector
+    # interning trims sel_registerName calls without changing what
+    # the program prints).
+    ir_file="$EXPECTED_DIR/${name}.ir"
+    actual_ir=""
+    has_ir_snapshot=false
+    if [[ -f "$ir_file" || ( $UPDATE -eq 1 && -f "$EXPECTED_DIR/${name}.ir" ) ]]; then
+        has_ir_snapshot=true
+    fi
+    if $has_ir_snapshot; then
+        actual_ir=$("$SX" ir "$sx_file" 2>&1 | normalize_ir)
+    fi
+
     if [[ $UPDATE -eq 1 ]]; then
         echo "$actual" > "$expected_file"
         echo "$actual_exit" > "$exit_file"
+        if $has_ir_snapshot; then
+            echo "$actual_ir" > "$ir_file"
+        fi
         echo "  updated $name (exit=$actual_exit)"
         continue
     fi
@@ -57,9 +90,14 @@ for expected_file in "$EXPECTED_DIR"/*.txt; do
     if [[ -f "$exit_file" ]]; then
         expected_exit=$(cat "$exit_file")
     fi
+    expected_ir=""
+    if $has_ir_snapshot; then
+        expected_ir=$(cat "$ir_file" | normalize_ir)
+    fi
 
     output_ok=true
     exit_ok=true
+    ir_ok=true
 
     if [[ "$actual" != "$expected" ]]; then
         output_ok=false
@@ -67,8 +105,11 @@ for expected_file in "$EXPECTED_DIR"/*.txt; do
     if [[ "$actual_exit" != "$expected_exit" ]]; then
         exit_ok=false
     fi
+    if $has_ir_snapshot && [[ "$actual_ir" != "$expected_ir" ]]; then
+        ir_ok=false
+    fi
 
-    if $output_ok && $exit_ok; then
+    if $output_ok && $exit_ok && $ir_ok; then
         PASS=$((PASS + 1))
         echo "ok"
     else
@@ -80,6 +121,10 @@ for expected_file in "$EXPECTED_DIR"/*.txt; do
         if ! $exit_ok; then
             echo "  exit code: expected=$expected_exit actual=$actual_exit"
         fi
+        if ! $ir_ok; then
+            echo "  IR diff:"
+            diff <(echo "$expected_ir") <(echo "$actual_ir") || true
+        fi
     fi
 done