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).

test/chez/corpus.edn

@@ -2981,4 +2981,8 @@
   {:suite "interop / instance? on host interfaces" :label "map is not a Collection" :expected "false" :actual "(instance? java.util.Collection {:a 1})"}
   {:suite "interop / instance? on host interfaces" :label "vector is Associative" :expected "true" :actual "(instance? clojure.lang.Associative [1])"}
   {:suite "interop / instance? on host interfaces" :label "string is not Number" :expected "false" :actual "(instance? java.lang.Number \"s\")"}
+  {:suite "interop / String & StringBuilder char ops" :label "String from char-array slice" :expected "\"abc\"" :actual "(String. (char-array [\\a \\b \\c \\d]) 0 3)"}
+  {:suite "interop / String & StringBuilder char ops" :label "str of StringBuilder" :expected "\"hi\"" :actual "(let [sb (StringBuilder.)] (.append sb \"hi\") (str sb))"}
+  {:suite "interop / String & StringBuilder char ops" :label "append subsequence" :expected "\"bcd\"" :actual "(let [sb (StringBuilder.)] (.append sb \"abcde\" 1 4) (.toString sb))"}
+  {:suite "interop / String & StringBuilder char ops" :label "String.getChars into buffer" :expected "\"abc\"" :actual "(let [a (char-array 4)] (.getChars \"abcd\" 0 3 a 0) (String. a 0 3))"}
 ]