Source locations: reader positions, error locations, native stack traces (#218)

* Reader records source line/column on list forms

The reader stamps 1-based :line/:column metadata on every list form (plus
:file when load-jolt-file is reading a file), and jolt.host/form-position
reads it back so the analyzer's :pos scaffold finally gets real data. A
left-to-right cursor counts newlines over the delta between successive forms,
so it stays O(n). Vector/map/set literals are untouched (their metadata is a
runtime value the analyzer would have to wrap in with-meta); empty () can't
carry meta. ^meta now merges onto the position keys instead of clobbering them.

Re-mint is byte-identical (the backend doesn't emit :pos), so this is a pure
scaffold for the error-location work that follows.

* Report source location on uncaught errors

Each top-level form records its source position (thread-local) before it
compiles+evals, and cli.ss jolt-report-uncaught appends 'at file:line:col'
when an error propagates out. Covers joltc -e, joltc run <file>, and
load-string — every interpreted path. Top-level granularity, one set per
form; deeper frames come from the Phase 2 frame walk.

Runtime .ss only, no re-mint.

* Clojure stack traces via source registry + native frame walk

A direct-link build emits (jolt-register-source! short-name ns name file line)
once per fn def — at definition time, so zero per-call cost. On an uncaught
error the reporter walks Chez's native continuation frames (jolt-throw captures
the live continuation via call/cc; host conditions carry their own
&continuation), maps each frame's procedure name through the registry, and
prints a Clojure backtrace 'ns/name (file:line)'. Wired into both the cli and a
built binary's launcher.

Frames are keyed by the short munged fn name Chez actually reports (emit-fn's
letrec self-binding), not jv$ns$name; a cross-namespace collision degrades to
the bare frame name rather than a wrong attribution. The analyzer carries the
original form's position through defn macroexpansion onto the def node.

Calling a non-fn now throws a catchable ClassCastException (via jolt-throw)
naming the operator, instead of a raw Chez error.

Caveats (documented in source-registry.ss): names map only in direct-link/AOT
closed-world builds — the open-world -e/repl/run path falls back to the
top-level location; and pervasive TCO erases tail-call frames, so a mapped
trace shows only the non-tail spine. JOLT_DEBUG_FRAMES dumps raw frame names.

Re-mint (analyzer + backend); prelude byte-identical (direct-link off during
mint). Corpus rows certified, build-smoke asserts the trace.

* Propagate source position through macroexpansion

hc-expand-1 now carries the macro call form's :line/:column onto the top of a
list expansion that has none of its own (merged under any meta the macro set),
so errors and stack traces in macro-generated code point at the call site —
Clojure parity. The analyze recursion re-expands inner macros, so each level's
top form picks it up, matching the reference compiler. (meta (macroexpand-1
'(when x y))) now reports the call-site line.

A direct-link fn defined through a user macro (build-app's defguarded) registers
with a real line, so build-smoke's trace assertion covers macro-defined fns.

Runtime .ss (host-contract.ss) — no re-mint; selfhost holds.

Phase 3's optional items are deferred: :line-in-ex-data has no clean consumer
(it would pollute ex-data, break = and printing, and positions already surface
via the trace + top-level location), and Chez source-object emission is a large
backend change the jv$-name registry already sidesteps.

* Review fixes: registration key, thread-locals, debug flag timing

- Register a fn under the name Chez actually reports for its frame, not the def
  name: a named fn literal whose name differs from the def (def foo (fn bar …))
  is framed as 'bar', and an anonymous fn def (def foo (fn …)) as jv$ns$foo.
  Both previously registered under the def name and so never appeared in traces.
- rdr-source-file / rdr-pos-cursor are thread parameters, so concurrent compiles
  (futures, core.async) don't clobber each other's file/line attribution.
- Read JOLT_DEBUG_FRAMES at call time: a built binary evaluates top-level forms
  at heap-build time, where a load-time getenv is always unset.

Re-mint (backend + reader); prelude byte-identical, selfhost holds.

---------

Co-authored-by: Yogthos <yogthos@gmail.com>

host/chez/compile-eval.ss

@@ -20,6 +20,29 @@
 ;; whose data conversion would turn those into real sets.
 (define jolt-ce-read jolt-read-form-raw)
 
+;; --- current source location ------------------------------------------------
+;; The position of the top-level form currently compiling/evaluating, so an
+;; uncaught error can report where it came from (cli.ss jolt-report-uncaught).
+;; Thread-local: a future/agent worker tracks its own form. Holds #f or a
+;; {:line :column :file?} position map (jolt.host/form-position's shape).
+;; Top-level granularity — one set per top-level form, nothing per call.
+(define jolt-current-source (make-thread-parameter #f))
+(define (jolt-enter-form! form)
+  (let ((p (hc-form-position form)))
+    (when (pmap? p) (jolt-current-source p))))
+
+;; "file:line:col" / "line:col" for the current form, or #f when none is set.
+(define (jolt-current-source-string)
+  (let ((p (jolt-current-source)))
+    (and (pmap? p)
+         (let ((line (jolt-get p hc-kw-line jolt-nil))
+               (col  (jolt-get p hc-kw-column jolt-nil))
+               (file (jolt-get p hc-kw-file jolt-nil)))
+           (string-append
+             (if (jolt-nil? file) "" (string-append file ":"))
+             (if (jolt-nil? line) "?" (number->string line)) ":"
+             (if (jolt-nil? col) "?" (number->string col)))))))
+
 ;; The spine ALWAYS runs with the full clojure.core prelude loaded, so a clojure.*
 ;; ref must lower to var-deref (resolved from the prelude), not trip the emitter's
 ;; "unsupported stdlib fn (no core on Chez yet)" out-of-subset guard — that guard
@@ -124,6 +147,9 @@
     ;; of the expander fn + (mark-macro! …) so subsequent forms expand it. One
     ;; macro-expansion path (no separate spine interception).
     (else
+     ;; record this form's source location first, so a compile- or run-time error
+     ;; in it reports the right place.
+     (jolt-enter-form! form)
      (eval (read (open-input-string (jolt-analyze-emit-form form ns)))
            (interaction-environment)))))