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/selfhost/selfhost_build.sh

@@ -24,6 +24,7 @@ fi
 
 IN=""
 JSON_OUT=""
+EXE_OUT=""
 DO_RUN=0
 
 while [ $# -gt 0 ]; do
@@ -31,6 +32,7 @@ while [ $# -gt 0 ]; do
     --in) IN="$2"; shift 2;;
     --json) JSON_OUT="$2"; shift 2;;
     --run) DO_RUN=1; shift;;
+    --exe) EXE_OUT="$2"; shift 2;;
     *) echo "[selfhost] unknown arg: $1" >&2; exit 2;;
   esac
 done
@@ -71,6 +73,26 @@ if [ -n "$JSON_OUT" ]; then
   echo "[selfhost] JSON v0 written: $tmp_json" >&2
 fi
 
+# Optional: build native EXE via ny-llvmc harness (fallback path; parses original source)
+if [ -n "$EXE_OUT" ]; then
+  # Requirements: ny-llvmc present and harness envs
+  NYLL="${NYASH_NY_LLVM_COMPILER:-$ROOT/target/release/ny-llvmc}"
+  if [ ! -x "$NYLL" ] && [ ! -f "$NYLL" ]; then
+    echo "[selfhost] ny-llvmc not found: $NYLL (skip EXE). Set NYASH_NY_LLVM_COMPILER or build ny-llvmc" >&2
+    exit 2
+  fi
+  NYRT_DIR="${NYASH_EMIT_EXE_NYRT:-$ROOT/target/release}"
+  export NYASH_LLVM_USE_HARNESS=1
+  export NYASH_NY_LLVM_COMPILER="$NYLL"
+  export NYASH_EMIT_EXE_NYRT="$NYRT_DIR"
+  echo "[selfhost] emitting EXE via ny-llvmc → $EXE_OUT" >&2
+  # Note: This path compiles the original Hako source via LLVM backend. Stage‑B JSON is emitted above for reference.
+  "$BIN" --backend llvm --emit-exe "$EXE_OUT" "$IN"
+  # Done
+  if [ -z "$JSON_OUT" ]; then rm -f "$tmp_json" 2>/dev/null || true; fi
+  exit 0
+fi
+
 if [ "$DO_RUN" = "1" ]; then
   # Run via Core‑Direct (in‑proc), quiet
   set +e
@@ -85,4 +107,3 @@ else
   # Emit-only
   echo "$tmp_json"
 fi
-