Selfhost build: add EXE path via ny-llvmc (tools) + smoke

- tools/selfhost/selfhost_build.sh: support --exe <out>, using LLVM harness (ny-llvmc)
- Add selfhost EXE canary (opt-in): selfhost_build_exe_return.sh
- Keep Stage‑B JSON emit for verification; EXE path currently parses original Hako source (80/20)

tools/smokes/v2/profiles/quick/selfhost/selfhost_build_exe_return.sh

@@ -0,0 +1,42 @@
+#!/bin/bash
+# selfhost_build_exe_return.sh — Build EXE via tools/selfhost_build.sh (opt‑in)
+
+set -euo pipefail
+if [ "${SMOKES_ENABLE_SELFHOST:-0}" != "1" ]; then
+  echo "[SKIP] selfhost_build_exe_return (SMOKES_ENABLE_SELFHOST=1 to enable)"
+  exit 0
+fi
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+ROOT="$(git -C "$SCRIPT_DIR" rev-parse --show-toplevel 2>/dev/null || cd "$SCRIPT_DIR/../../../../../../.." && pwd)"
+source "$ROOT/tools/smokes/v2/lib/test_runner.sh"
+require_env || exit 2
+
+# Check ny-llvmc
+NYLL="${NYASH_NY_LLVM_COMPILER:-$ROOT/target/release/ny-llvmc}"
+if [ ! -f "$NYLL" ]; then
+  echo "[SKIP] selfhost_build_exe_return (ny-llvmc not found)"
+  exit 0
+fi
+
+tmp_hako="/tmp/selfhost_exe_return7_$$.hako"
+tmp_exe="/tmp/selfhost_exe_return7_$$.out"
+cat > "$tmp_hako" <<'HAKO'
+static box Main { method main(args) { return 7 } }
+HAKO
+
+"$ROOT/tools/selfhost/selfhost_build.sh" --in "$tmp_hako" --exe "$tmp_exe" >/dev/null 2>&1 || { echo "[FAIL] selfhost_build_exe_return (build failed)" >&2; exit 1; }
+
+set +e
+"$tmp_exe" >/dev/null 2>&1
+rc=$?
+set -e
+rm -f "$tmp_hako" "$tmp_exe" 2>/dev/null || true
+
+if [ "$rc" = "7" ]; then
+  echo "[PASS] selfhost_build_exe_return"
+else
+  echo "[FAIL] selfhost_build_exe_return (rc=$rc)" >&2
+  exit 1
+fi
+