Chez Phase 2 (inc Y): runtime data reader + clojure.edn (jolt-r8ku)

Chez-side recursive-descent Clojure reader (host/chez/reader.ss) producing the
same jolt forms the Janet reader yields, behind the read-string / __parse-next /
__read-tagged seams the seed registers in eval_runtime.janet. That lights up the
whole *in* read family — read, read+string, with-in-str (read) — plus read-string
and read-string metadata, none of which needed an analyzer change (read-string is
a clojure.core seam, jolt-nil on the prelude until now).

Reader output is pinned to the Janet reader's shapes: numbers coerce to flonum
(the all-double model emit-const uses, else a read int isn't = a source int),
sets read as the {:jolt/type :jolt/set :value [...]} FORM, #tag/#inst/#uuid/#regex
as tagged forms (no data reader applied — read-string never evaluates), ^meta on
a symbol, and the ' ` ~ ~@ @ reader macros. clojure.edn is added to the prelude
tier; its edn->value builds the real set/tagged values and __read-tagged reuses
the inc X #inst/#uuid constructors. The reader-arity edn/read stays a lazy gap
(drain-reader is Janet-coupled) — read-string is the live path.

eval / load-string / runtime defmacro are still out: they need the compiler at
runtime, which is Phase 3 (self-host). Chez-only change, no Janet gate.

Parity 2238 -> 2259, 0 new divergences. _reader 47/47; all chez unit tests green;
emit-test 331/331.

host/chez/rt.ss

@@ -299,3 +299,8 @@
 ;; LAST — it extends record-method-dispatch / jolt-get / jolt= / jolt-hash /
 ;; jolt-pr-str / jolt-type / instance-check and uses host-static.ss's registries.
 (load "host/chez/inst-time.ss")
+
+;; Chez-side data reader (jolt-r8ku inc Y): read-string / __parse-next /
+;; __read-tagged. Loads after inst-time.ss — __read-tagged reuses its #uuid/#inst
+;; constructors, and the reader needs the full value/collection layer above.
+(load "host/chez/reader.ss")