data.csv: fully passes, three general fixes

clojure.data.csv runs its whole suite on jolt (4/4 reading/writing/eof/line-
endings). Three general gaps fixed, all runtime, no re-mint, JVM-certified:

- The prefix-list form of :require/:use — (:require (clojure [string :as str]))
  means clojure.string :as str — now expands (loader.ss). It silently failed
  before, trying to load a "clojure" namespace.
- extend-protocol to java.io.Reader / Writer / StringReader / PushbackReader now
  dispatches: those reader/writer host tags carry the right class names in
  value-host-tags AND are in host-type-set, so extend-protocol registers under
  the canonical tag instead of a local ns tag (records.ss). data.csv's
  Read-CSV-From protocol extends to String / Reader / PushbackReader.
- (str StringWriter) returns its accumulated content (register-str-render for the
  "writer" jhost), not the opaque host object — data.csv writes CSV to one and
  reads it back.

Listed in docs/libraries.md + the site.

make test green (+2 corpus rows, 0 new divergences), shakesmoke byte-identical.

host/chez/java/host-static-classes.ss

@@ -154,6 +154,9 @@
         (cons "flush" (lambda (self) jolt-nil))
         (cons "close" (lambda (self) jolt-nil))
         (cons "toString" (lambda (self) (sb-str self)))))
+;; (str sw) / print a StringWriter -> its accumulated content, like the JVM
+;; (str calls toString) — data.csv writes CSV to a StringWriter and reads it back.
+(register-str-render! (lambda (x) (and (jhost? x) (string=? (jhost-tag x) "writer"))) sb-str)
 
 ;; a file-backed writer (clojure.java.io/writer of a File/path): accumulates like
 ;; StringWriter, then persists to the path on flush/close, so