*in* is a Reader, not a map

(map? *in*) was true because *in* was a plain map of read-line-fn/read-fn
closures; the JVM *in* is a java.io.Reader so map? is false. A defrecord
doesn't help (records are maps). Make the reader a reify over a new IReader
protocol — a non-map value — and route read/read-line/read+string/line-seq
through its -read-line/-read-form/-read+string methods instead of keyword
access. with-in-str's __string-reader and the stdin *in* both reify it.
Closes *in*-bound + *in*-is-bound.

host/chez/run-corpus.ss

@@ -11,7 +11,7 @@
 ;; reset between cases so there is no leakage — same isolation a fresh process gives.
 ;;
 ;;   chez --script host/chez/run-corpus.ss
-;;   JOLT_CHEZ_ZJ_FLOOR=N   override the regression floor (default 2723)
+;;   JOLT_CHEZ_ZJ_FLOOR=N   override the regression floor (default 2725)
 ;;   JOLT_CORPUS_LIMIT=N    every-Nth stride (fast iteration; floor drops to 0)
 ;;   JOLT_DUMP_CRASH_LABELS=1   list crash + allowlisted labels
 (import (chezscheme))
@@ -91,7 +91,6 @@
     "^Type tag on var"
     "close on throw" "macroexpand-1"
     "bean is the map" "proxy resolves nil"
-    "*in* is bound" "*in* bound"
     "bigdec" "bigdec int M" "bigdec suffix M"
     "transient vector" "transient map"
     "atom override fires nested"
@@ -192,7 +191,7 @@
 
 ;; Regression floor: fail on any NEW divergence or if pass drops below the floor.
 (define base-floor (let ((s (getenv "JOLT_CHEZ_ZJ_FLOOR")))
-                     (if s (string->number s) 2723)))
+                     (if s (string->number s) 2725)))
 (define floor (if limit 0 base-floor))
 (when (or (> (length diverged) 0) (< pass floor))
   (printf "REGRESSION: pass ~a < floor ~a or ~a new divergence(s)\n"