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/cli.ss

@@ -32,27 +32,17 @@
 (set-source-roots! (list "jolt-core" "stdlib"))
 
 ;; Render an uncaught jolt throw (any value, not just a Chez condition) to stderr
-;; and exit non-zero, instead of Chez's opaque "non-condition value" dump. An
-;; ex-info shows its message + ex-data; anything else is pr-str'd.
+;; and exit non-zero, instead of Chez's opaque "non-condition value" dump. The
+;; message/ex-data/cause + a mapped Clojure backtrace come from the shared
+;; renderer (source-registry.ss); the cli adds the top-level source location.
 (define (jolt-report-uncaught v)
   (let ((port (current-error-port)))
-    (if (and (jolt=2 (jolt-get v jolt-kw-ex-type jolt-nil) jolt-kw-ex-info))
-        (begin
-          (display "Unhandled exception: " port)
-          (display (jolt-str-render-one (jolt-get v jolt-kw-message jolt-nil)) port)
-          (newline port)
-          (let ((data (jolt-get v jolt-kw-data jolt-nil)))
-            (unless (jolt-nil? data)
-              (display "  ex-data: " port) (display (jolt-pr-str data) port) (newline port)))
-          (let ((cause (jolt-get v jolt-kw-cause jolt-nil)))
-            (when (condition? cause)
-              (display "  cause: " port)
-              (display (with-output-to-string (lambda () (display-condition cause))) port)
-              (newline port))))
-        (begin
-          (display "Unhandled exception: " port)
-          (display (if (condition? v) (with-output-to-string (lambda () (display-condition v))) (jolt-pr-str v)) port)
-          (newline port)))
+    (jolt-render-throwable v port)
+    ;; The top-level form that was evaluating when this propagated (file:line:col).
+    (let ((loc (jolt-current-source-string)))
+      (when loc (display "  at " port) (display loc port) (newline port)))
+    (let ((bt (jolt-backtrace-string v)))
+      (when bt (display "  trace:\n" port) (display bt port)))
     (exit 1)))
 
 (guard (v (#t (jolt-report-uncaught v)))