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.

docs/libraries.md

@@ -60,6 +60,7 @@ e.g. the [ring-app example](https://github.com/jolt-lang/examples/tree/main/ring
 * [data.zip](https://github.com/clojure/data.zip) — zipper navigation, including
   `clojure.data.zip.xml`; XML parsing via [jolt-lang/xml](https://github.com/jolt-lang/xml)
   (which now ships `clojure.xml/parse`).
+* [data.csv](https://github.com/clojure/data.csv) — reading and writing CSV.
 * [tick](https://github.com/juxt/tick) — date/time over Jolt's `java.time`;
   `#time/…` literals via `time-literals`.
 * [transit-jolt](https://github.com/jolt-lang/transit-jolt) — Transit (JSON) read/write