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/host-contract.ss

@@ -119,8 +119,22 @@
 (define (hc-inst-source x) (jolt-get x hc-kw-form))
 (define (hc-uuid-source x) (jolt-get x hc-kw-form))
 
-;; The Chez reader does not record source offsets yet.
-(define (hc-form-position x) jolt-nil)
+;; Source position for a list form: the reader stamps :line/:column (+ :file when
+;; compiling a file) into the form's metadata. Return a clean {:line :column
+;; :file?} map, or nil for a synthetic/macro-built form that carries none.
+(define hc-kw-line   (keyword #f "line"))
+(define hc-kw-column (keyword #f "column"))
+(define hc-kw-file   (keyword #f "file"))
+(define (hc-form-position x)
+  (let ((m (jolt-meta x)))
+    (if (and (pmap? m) (not (jolt-nil? (jolt-get m hc-kw-line))))
+        (let ((line (jolt-get m hc-kw-line))
+              (col  (jolt-get m hc-kw-column))
+              (file (jolt-get m hc-kw-file)))
+          (if (jolt-nil? file)
+              (jolt-hash-map hc-kw-line line hc-kw-column col)
+              (jolt-hash-map hc-kw-line line hc-kw-column col hc-kw-file file)))
+        jolt-nil)))
 
 ;; --- special forms ----------------------------------------------------------
 ;; Mirrors host_iface special-names + interop-head? — forms the analyzer marks
@@ -170,12 +184,31 @@
 ;; of the list), and the analyzer re-analyzes the returned form.
 (define (hc-macro? ctx sym)
   (macro-var? (hc-resolve-cell ctx sym)))
+;; Clojure parity: a macro expansion inherits the call form's source position, so
+;; errors/traces in macro-generated code point at the macro call site. Carry it
+;; onto the top of a LIST expansion (code) that has none of its own — merged under
+;; any meta the macro set, leaving collection literals (runtime data) alone. The
+;; recursion through analyze re-expands inner macros, so each level's top form
+;; picks up the position the same way (as the reference compiler does).
+(define (hc-propagate-pos src dst)
+  (if (and (cseq? dst) (cseq-list? dst))
+      (let ((sp (hc-form-position src))
+            (dm (jolt-meta dst)))
+        (if (and (pmap? sp)
+                 (or (jolt-nil? dm) (jolt-nil? (jolt-get dm hc-kw-line))))
+            (jolt-with-meta dst
+              (if (pmap? dm)
+                  (pmap-fold sp (lambda (k v acc) (jolt-assoc1 acc k v)) dm)
+                  sp))
+            dst))
+      dst))
+
 (define (hc-expand-1 ctx form)
   (let* ((items (seq->list form))
          (head (car items))
          (args (cdr items))
          (expander (var-cell-root (hc-resolve-cell ctx head))))
-    (apply jolt-invoke expander args)))
+    (hc-propagate-pos form (apply jolt-invoke expander args))))
 
 ;; Classify a global (non-local) symbol reference against the var registry:
 ;;   {:kind :var :ns NS :name NAME}   — a defined var (compile ns / clojure.core)