Selfhost: prefer BuildBox for emit-only (opt-in) and fix selfhost smokes root detection

- selfhost_build.sh: HAKO_USE_BUILDBOX=1 uses BuildBox.emit_program_json_v0 for emit-only paths
- Fix ROOT detection in selfhost and bridge negative smokes
- All selfhost smokes pass (opt-in)

tools/selfhost/selfhost_build.sh

@@ -42,10 +42,28 @@ if [ ! -f "$IN" ]; then echo "[selfhost] input not found: $IN" >&2; exit 2; fi
 
 tmp_json="${JSON_OUT:-/tmp/hako_stageb_$$.json}"
 
-# Stage‑B emit (Program JSON v0; single line)
+# Emit Program(JSON v0; prefer BuildBox for emit-only when HAKO_USE_BUILDBOX=1)
 RAW="/tmp/hako_stageb_raw_$$.txt"
 SRC_CONTENT="$(cat "$IN")"
-(
+if [ "${HAKO_USE_BUILDBOX:-0}" = "1" ] && [ "$DO_RUN" = "0" ] && [ -z "$EXE_OUT" ]; then
+  WRAP="/tmp/hako_buildbox_wrap_$$.hako"
+  cat > "$WRAP" <<'HAKO'
+include "lang/src/compiler/build/build_box.hako"
+static box Main { method main(args) {
+  local src = env.get("HAKO_SRC");
+  local j = BuildBox.emit_program_json_v0(src, null);
+  print(j);
+  return 0;
+} }
+HAKO
+  (
+    export HAKO_SRC="$SRC_CONTENT"
+    export NYASH_QUIET=0 HAKO_QUIET=0 NYASH_CLI_VERBOSE=0
+    cd "$ROOT" && "$BIN" --backend vm "$WRAP"
+  ) > "$RAW" 2>&1 || true
+  rm -f "$WRAP" 2>/dev/null || true
+else
+  (
     export NYASH_PARSER_ALLOW_SEMICOLON=1
     export NYASH_ALLOW_USING_FILE=0
     export HAKO_ALLOW_USING_FILE=0
@@ -59,7 +77,8 @@ SRC_CONTENT="$(cat "$IN")"
       "$BIN" --backend vm \
         "$ROOT/lang/src/compiler/entry/compiler_stageb.hako" -- \
         --source "$SRC_CONTENT"
-) > "$RAW" 2>&1 || true
+  ) > "$RAW" 2>&1 || true
+fi
 
 if ! awk '(/"version":0/ && /"kind":"Program"/){print;found=1;exit} END{exit(found?0:1)}' "$RAW" > "$tmp_json"; then
   echo "[selfhost] Stage‑B emit failed" >&2

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

@@ -7,7 +7,11 @@ if [ "${SMOKES_ENABLE_SELFHOST:-0}" != "1" ]; then
   exit 0
 fi
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
-ROOT="$(git -C "$SCRIPT_DIR" rev-parse --show-toplevel 2>/dev/null || cd "$SCRIPT_DIR/../../../../../../.." && pwd)"
+if ROOT_GIT=$(git -C "$SCRIPT_DIR" rev-parse --show-toplevel 2>/dev/null); then
+  ROOT="$ROOT_GIT"
+else
+  ROOT="$(cd "$SCRIPT_DIR/../../../../../../../../.." && pwd)"
+fi
 source "$ROOT/tools/smokes/v2/lib/test_runner.sh"
 require_env || exit 2
 

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

@@ -7,7 +7,11 @@ if [ "${SMOKES_ENABLE_SELFHOST:-0}" != "1" ]; then
   exit 0
 fi
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
-ROOT="$(git -C "$SCRIPT_DIR" rev-parse --show-toplevel 2>/dev/null || cd "$SCRIPT_DIR/../../../../../../.." && pwd)"
+if ROOT_GIT=$(git -C "$SCRIPT_DIR" rev-parse --show-toplevel 2>/dev/null); then
+  ROOT="$ROOT_GIT"
+else
+  ROOT="$(cd "$SCRIPT_DIR/../../../../../../../../.." && pwd)"
+fi
 source "$ROOT/tools/smokes/v2/lib/test_runner.sh"
 require_env || exit 2