ERR/E3.0 (slice 3d): --emit-obj + macOS lldb stepping verified

`sx build --emit-obj` keeps the DWARF-bearing object so a debugger can
step the binary, completing the deep-debug half of the trace story.

- --emit-obj flag + TargetConfig.emit_obj. Implies -O0 (DWARF only
  emits at opt none/less); keeps the object at its link-time path
  .sx-tmp/main.o so the binary's debug map resolves to it; skips the
  Level-1 binary cache; reports the object path. macOS resolves via the
  debug map -> .o; Linux carries DWARF in the binary. Build-flow only,
  no runtime/codegen change.
- tests/debug_stepping_smoke.sh (3e rung 1; macOS, lldb, not in
  run_examples): builds with --emit-obj, drives an lldb file:line
  breakpoint, asserts resolution + a source-mapped backtrace. Passing —
  proves the slice 1-2 DWARF drives real source-level stepping.

(Also normalizes the 253 .exit trailing newline from the 3c --update.)
Gates: zig build, zig build test, run_examples.sh -> 291 passed.

tests/debug_stepping_smoke.sh

@@ -0,0 +1,54 @@
+#!/usr/bin/env bash
+# Debug-stepping smoke (ERR E3.0 slice 3e, rung 1: macOS native).
+#
+# Verifies the DWARF emitted by `sx build --emit-obj` actually drives
+# source-level stepping in lldb — the deep-debug half of the trace story.
+# NOT part of `run_examples.sh` (it needs `lldb`, and is macOS-specific via the
+# debug-map → kept `.o`). Run manually:  bash tests/debug_stepping_smoke.sh
+#
+# Exit 0 = lldb resolved a file:line breakpoint + a source-mapped backtrace.
+
+set -u
+ROOT_DIR="$(cd "$(dirname "$0")/.." && pwd)"
+SX="$ROOT_DIR/zig-out/bin/sx"
+TMP="$ROOT_DIR/.sx-tmp"
+SRC="$TMP/dbg_smoke.sx"
+BIN="$TMP/dbg_smoke"
+
+if ! command -v lldb >/dev/null 2>&1; then
+    echo "SKIP: lldb not found (macOS/Xcode tools required)"
+    exit 0
+fi
+
+mkdir -p "$TMP"
+cat > "$SRC" <<'EOF'
+add :: (a: s32, b: s32) -> s32 {
+    c := a + b;
+    return c;
+}
+main :: () -> s32 {
+    return add(40, 2);
+}
+EOF
+
+"$SX" build --emit-obj "$SRC" -o "$BIN" >/dev/null 2>&1 || { echo "FAIL: build"; exit 1; }
+
+# Breakpoint on the `return c;` line; expect lldb to resolve it + a backtrace
+# mapping both frames to dbg_smoke.sx.
+out=$(cd "$ROOT_DIR" && lldb --batch \
+    -o "breakpoint set --file dbg_smoke.sx --line 3" \
+    -o "run" -o "bt" -o "quit" "$BIN" 2>&1)
+
+rm -f "$SRC" "$BIN" "$TMP/main.o"
+
+fail=0
+echo "$out" | grep -q "dbg_smoke.sx:3" || { echo "FAIL: breakpoint did not resolve to dbg_smoke.sx:3"; fail=1; }
+echo "$out" | grep -q "add at dbg_smoke.sx:3" || { echo "FAIL: stopped frame not source-mapped"; fail=1; }
+echo "$out" | grep -q "main at dbg_smoke.sx:6" || { echo "FAIL: caller frame not source-mapped"; fail=1; }
+
+if [[ $fail -eq 0 ]]; then
+    echo "ok: lldb stepped sx source (breakpoint + backtrace resolved via DWARF)"
+    exit 0
+fi
+echo "--- lldb output ---"; echo "$out"
+exit 1

tests/expected/253-comptime-trace.exit

@@ -1 +1 @@
-0
+0