build: a no-main entry namespace runs as a script instead of crashing

jolt build -m ns on a namespace with only top-level side effects and no
-main produced a binary that printed its output, then crashed calling a nil
-main ("nil cannot be cast to IFn"). The launcher now calls -main only when
the entry ns actually defines one; otherwise the top-level forms (already run
at heap build) are the whole program and it exits cleanly. Regression case
added to build-smoke.sh.

host/chez/build-smoke.sh

@@ -129,4 +129,20 @@ if [ "$got_dr" != "42" ]; then
   echo "  FAIL: built #code data reader — want 42, got \`$got_dr\`"; exit 1
 fi
 
-echo "build smoke: passed (release + optimized + direct-link + tree-shake + compiler+core shake + data-reader)"
+# A script namespace with no -main (just top-level side effects) must build and
+# run its top-level forms, then exit cleanly — not crash calling a nil -main.
+nomain="$(dirname "$out")/nomain"
+mkdir -p "$nomain/src"
+printf '{:paths ["src"]}\n' > "$nomain/deps.edn"
+printf '(ns script)\n(println "no-main script ran")\n' > "$nomain/src/script.clj"
+nmout="$(dirname "$out")/nomain-bin"
+if ! JOLT_PWD="$nomain" bin/joltc build -m script -o "$nmout" >/dev/null 2>&1; then
+  echo "  FAIL: jolt build of a no-main script exited non-zero"; exit 1
+fi
+got_nm="$(cd / && "$nmout" 2>&1)"; rc_nm=$?
+if [ "$got_nm" != "no-main script ran" ] || [ "$rc_nm" != "0" ]; then
+  echo "  FAIL: no-main script binary — want 'no-main script ran' rc 0, got \`$got_nm\` rc $rc_nm"
+  exit 1
+fi
+
+echo "build smoke: passed (release + optimized + direct-link + tree-shake + compiler+core shake + data-reader + no-main)"

host/chez/build.ss

@@ -571,11 +571,17 @@
                             "))\n"
                             "                (list \"jolt-core\" \"stdlib\"))))\n"))
           (put-string out (string-append
-                            "    (let ((mainv (var-deref " (ei-str-lit entry-ns) " \"-main\")))\n"
+                            ;; Call -main only if the entry namespace defines one;
+                            ;; a script ns (top-level side effects, no -main) has
+                            ;; already run its forms at heap build, so invoking a nil
+                            ;; -main would crash ("nil cannot be cast to IFn") — just
+                            ;; exit cleanly instead.
+                            "    (let ((maincell (var-cell-lookup " (ei-str-lit entry-ns) " \"-main\")))\n"
                             ;; render an uncaught throw (+ Clojure backtrace) instead
                             ;; of Chez's opaque dump, then exit non-zero.
                             "      (guard (v (#t (jolt-report-throwable v (current-error-port)) (exit 1)))\n"
-                            "        (apply jolt-invoke mainv args)))\n"
+                            "        (when (and maincell (var-cell-defined? maincell))\n"
+                            "          (apply jolt-invoke (var-cell-root maincell) args))))\n"
                             "    (exit 0)))\n"))
           (close-port out))
         ;; 4. compile -> boot -> link. Two paths, chosen by whether this process