Recover TCO-elided frames in uncaught-error stack traces

On the eval path nothing registers a source map, so jolt-backtrace-string
dropped every walkable frame and printed no trace at all. Keep any named,
non-plumbing continuation frame (rendered as a bare name when unmapped) so a
runtime error shows the surviving non-tail spine — "print what is available".

Add an opt-in tail-frame history behind JOLT_TRACE for the frames TCO erases.
Each compiled fn records itself on entry into a bounded ring-of-rings, MIT
Scheme's "history" shape: the outer ring holds one rib per non-tail subproblem,
each rib a small inner ring of the tail-calls made at that level. A tight tail
loop churns one rib instead of flushing the spine, so the non-tail caller
context survives and total space stays bounded. The reporter prefers this
history over the continuation when it's present, and resets it per top-level
form so an error's trace isn't padded with earlier REPL frames.

The emitter marks a tail call with (jolt-trace-mark! #t) so the runtime routes
the callee into the current rib vs a fresh one; a *tail?* dynamic var tracks
tail position (cleared by default, passed through if/do/let/loop/fn-body). It's
all gated on trace-frames?, which compile-eval turns on for JOLT_TRACE and
emit-image/`jolt build` force off — so non-trace emitted output is byte-identical
(prelude unchanged, seed re-minted), and a built binary carries no per-call cost.

host/chez/compile-eval.ss

@@ -110,6 +110,13 @@
 ;; older seed during the first re-mint pass.
 (let ((scv (var-deref "jolt.backend-scheme" "set-var-cache!")))
   (when (procedure? scv) (scv #t)))
+;; JOLT_TRACE opts into tail-frame history: the emitter prepends a frame-recording
+;; push to every runtime-compiled fn, and the ring buffer is allocated for this
+;; thread. Off by default, so a normal run emits and pays exactly as before.
+(when (getenv "JOLT_TRACE")
+  (let ((stf (var-deref "jolt.backend-scheme" "set-trace-frames!")))
+    (when (procedure? stf) (stf #t)))
+  (jolt-trace-enable!))
 
 ;; (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.
@@ -235,6 +242,9 @@
      ;; record this form's source location first, so a compile- or run-time error
      ;; in it reports the right place.
      (jolt-enter-form! form)
+     ;; drop tail-frame history from earlier top-level forms, so an error's trace
+     ;; shows only this form's own call history (a no-op unless JOLT_TRACE is on).
+     (jolt-trace-reset!)
      (eval (read (open-input-string (jolt-analyze-emit-form form ns)))
            (interaction-environment)))))