JOLT_TRACE: honor the env at runtime in a built joltc

The JOLT_TRACE opt-in was a top-level form in compile-eval.ss, so in a
self-contained joltc it ran at heap-build time — where JOLT_TRACE is always
unset — and never at runtime. `JOLT_TRACE=1 joltc -M:run` therefore produced no
trace from the distributed binary (it worked only under the source-loaded dev
launcher). REPL/nREPL tracing was unaffected (those enable at runtime).

Make it a jolt-trace-init-from-env! fn called from the runtime entrypoints — the
cli.ss dispatch and the built-joltc launcher — before any app namespace compiles,
so the app's own code is traced. While here, drop a redundant trace print in the
joltc launcher (jolt-report-throwable already emits it) that double-printed the
block once tracing actually produced one.

joltc-selfbuild-smoke asserts JOLT_TRACE=1 through the built binary yields exactly
one tail-frame trace.

host/chez/build-joltc.ss

@@ -145,11 +145,10 @@
 (scheme-start
   (lambda args
     (set-source-roots! (list \"jolt-core\" \"stdlib\"))
-    (guard (v (#t (jolt-report-throwable v (current-error-port))
-                  (let ((bt (jolt-backtrace-string v)))
-                    (when bt (display \"  trace:\\n\" (current-error-port))
-                          (display bt (current-error-port))))
-                  (exit 1)))
+    ;; JOLT_TRACE at RUNTIME (the env is unset at heap-build), before any app ns
+    ;; compiles, so a `-M:run` traces the app's own code.
+    (jolt-trace-init-from-env!)
+    (guard (v (#t (jolt-report-throwable v (current-error-port)) (exit 1)))
       (cond
         ((and (= (length args) 2) (string=? (car args) \"-e\"))
          (let ((result (jolt-final-str

host/chez/cli.ss

@@ -66,6 +66,9 @@
       (when bt (display "  trace:\n" port) (display bt port)))
     (exit 1)))
 
+;; JOLT_TRACE opt-in, at runtime (before any app ns compiles) so the app is traced.
+(jolt-trace-init-from-env!)
+
 (guard (v (#t (jolt-report-uncaught v)))
   (cond
     ;; -e EXPR — evaluate one expression and print it (blank for nil). Wrapped in

host/chez/compile-eval.ss

@@ -132,11 +132,16 @@
 ;; push is baked in at compile time, only code compiled after this call is traced —
 ;; which is exactly the code you eval / reload in a live session.
 (def-var! "jolt.host" "enable-trace!" jolt-enable-trace!)
-;; Explicit opt-in for a whole run (JOLT_TRACE=1): enable at load, BEFORE any app
-;; namespace is compiled, so a plain `-M:run` traces the app's own code too. Only an
-;; affirmative value (set, non-empty, not falsey) forces it on here.
-(let ((e (getenv "JOLT_TRACE")))
-  (when (and e (fx>? (string-length e) 0) (not (jolt-trace-env-off? e))) (jolt-enable-trace!)))
+;; Explicit opt-in for a whole run (JOLT_TRACE=1): turn tracing on BEFORE any app
+;; namespace is compiled, so a plain `-M:run` traces the app's own code too. Called
+;; from the runtime entrypoints (cli.ss, and the built joltc launcher) — NOT at load
+;; time: a built joltc runs top-level forms at heap-build time, where JOLT_TRACE is
+;; always unset, so a load-time check would never see the user's runtime env. Only an
+;; affirmative value (set, non-empty, not falsey) forces it on.
+(define (jolt-trace-init-from-env!)
+  (let ((e (getenv "JOLT_TRACE")))
+    (when (and e (fx>? (string-length e) 0) (not (jolt-trace-env-off? e)))
+      (jolt-enable-trace!))))
 
 ;; (with-meta sym m) -> sym, else x — an (ns ^:no-doc name …) yields the name with
 ;; reader metadata as a with-meta form; strip it to read the bare ns symbol.

host/chez/joltc-selfbuild-smoke.sh

@@ -42,6 +42,20 @@ if [ "$got_e" != "45" ]; then
   exit 1
 fi
 
+# 2b. JOLT_TRACE must take effect in the BUILT binary. The env check runs at
+# runtime (the launcher), NOT at heap-build where JOLT_TRACE is always unset — so
+# an uncaught error shows a tail-frame trace recovering the TCO-elided chain, and
+# exactly ONE trace block (the launcher must not double-print it).
+got_tr="$(env -i HOME="$HOME" JOLT_TRACE=1 "$joltc" -e '(defn a [x] (+ x 1)) (defn b [x] (a x)) (b :x)' 2>&1)"
+if ! printf '%s' "$got_tr" | grep -q '  trace:' || ! printf '%s' "$got_tr" | grep -q 'b'; then
+  echo "  FAIL: JOLT_TRACE=1 in the built joltc produced no tail-frame trace"
+  echo "--- got ---"; echo "$got_tr"; exit 1
+fi
+if [ "$(printf '%s' "$got_tr" | grep -c '  trace:')" != "1" ]; then
+  echo "  FAIL: built joltc double-printed the trace block"
+  echo "--- got ---"; echo "$got_tr"; exit 1
+fi
+
 # 3. Build an app through the distributed joltc with an EMPTY environment — no
 # PATH at all, so no chez, no cc, no shell tools are reachable. This is the core
 # guarantee: joltc compiles apps entirely on its own.