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.
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