Make the REPL read multi-line forms and render real error messages

The REPL evaluated one line at a time, so a form split across lines
(e.g. `(+` then `1 2)`) raised instead of waiting. The read loop now
accumulates lines until delimiters are balanced — skipping string,
char, regex and comment context — printing a `... ` continuation prompt
for each extra line.

Reader/runtime errors rendered as Chez's "attempt to apply
non-procedure #[chez-pmap...]" instead of their real message. Two causes:

jolt-throw raised the thrown value raw. When a throw crossed the host
`eval` boundary, Chez re-wrapped the non-condition into a compound
condition whose message extraction applies the value, losing the message
and crashing on ex-info's empty-map :data. jolt-throw now raises a
&jolt-throw condition wrapping the value; catch (lowered to `guard`),
jolt-report-uncaught and jolt-render-throwable unwrap it back via
jolt-unwrap-throw, so ex-data/ex-message and the backtrace tag survive.

Every reader/post-prelude EOF-throw site used `(empty-pmap)` (with
parens), applying the empty-map value as a procedure and crashing during
ex-info construction before jolt-throw ran. Fixed to `empty-pmap`.

Re-minted the seed; smoke 23/23, unit 574/574.

host/chez/cli.ss

@@ -35,8 +35,9 @@
 ;; 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)))
+(define (jolt-report-uncaught raw)
+  (let ((v (jolt-unwrap-throw raw))
+        (port (current-error-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)))