String/char-array interop: getChars, String(char[]), str of StringBuilder, append(csq,start,end)

- String.getChars copies into a char-array; String. builds from a char[]
  (whole or offset/count slice); subSequence returns the substring.
- str of a StringBuilder returns its content (was the opaque host object;
  .toString already worked).
- Appendable.append gains the 3-arg subsequence form append(csq,start,end)
  on StringBuilder/StringWriter/file/port writers.
- reader combines a \uXXXX surrogate pair into the one Unicode scalar
  (😃) instead of crashing on the lone high surrogate; a stray surrogate
  maps to U+FFFD. (A high-plane char re-escaped as \uXXXXX stays the
  irreducible UTF-16/scalar divergence.)
- TimeZone/getDefault.

These let clojure.data.json read and write JSON; its own suite goes from
not loading to 97/133 assertions passing (remainder: per-type writer
dispatch for uuid/bigdec/date, EOFException, niche date interop).

host/chez/inst-time.ss

@@ -356,8 +356,11 @@
 ;; java.util.TimeZone: an opaque id holder (format-ms is UTC, so a non-UTC zone is
 ;; not honored — only the UTC case the corpus uses is exercised).
 (define (timezone-of id) (make-jhost "timezone" (vector (if (string? id) id (jolt-str-render-one id)))))
-(register-class-statics! "TimeZone" (list (cons "getTimeZone" timezone-of)))
-(register-class-statics! "java.util.TimeZone" (list (cons "getTimeZone" timezone-of)))
+(define timezone-statics
+  (list (cons "getTimeZone" timezone-of)
+        (cons "getDefault" (lambda () (timezone-of "default")))))
+(register-class-statics! "TimeZone" timezone-statics)
+(register-class-statics! "java.util.TimeZone" timezone-statics)
 
 ;; java.text.SimpleDateFormat: holds a pattern; .setTimeZone is accepted (format-ms
 ;; is UTC); .format(date) renders the date per the pattern via the format-ms engine.