host: enable reitit — runtime JOLT_FEATURES, class-shim hooks, get-on-strings, java shims

Everything reitit-core needs to load unmodified from git under :clj features:

- The baked binary now re-reads JOLT_FEATURES at startup (like JOLT_PATH).
  reader-features-set! runs at module load = BUILD time for a binary, so a
  process opting into :clj (to read a lib's :clj branches) was ignored, and
  unmatched #?(...) forms silently spliced to nothing — defn of a fn with an
  empty arglist, hence the cryptic index errors.

- (get s i) indexes a string and returns the char, as in Clojure (nth did;
  get returned nil). reitit's path parser is (get path i)-based — without
  this every route read as static.

- Class-shim registration exposed to Clojure: __register-class-statics! /
  __register-class-methods! / __register-class-ctor!, so a library can mirror
  a Java class jolt doesn't ship (the reitit.Trie mirror lives in jolt-lang/
  router on top of these).

- Java surface reitit's :clj branches call: .getMessage (on exceptions and
  strings) and a small universal object-method set, .intern, java.util.HashMap
  (a mutable map wrapper). Plus defprotocol already took keyword options.

Gate green; clojure-test-suite 4715 -> 4718 (the get-on-strings fix).

docs/libraries.md

@@ -12,6 +12,10 @@ Libraries confirmed to load and pass their conformance checks on Jolt
   on the [ring-app example](https://github.com/jolt-lang/examples/tree/main/ring-app)'s
   spork/http adapter
 * [ring-codec](https://github.com/ring-clojure/ring-codec)
+* [reitit-core](https://github.com/metosin/reitit) — data-driven routing; the
+  reitit.Trie Java class is mirrored in Clojure by
+  [jolt-lang/router](https://github.com/jolt-lang/router). Load with
+  `JOLT_FEATURES` including `clj`.
 * [honeysql](https://github.com/seancorfield/honeysql) — full formatter + helpers
   (select/insert/update/delete/joins/:inline), loaded unmodified from git
 * [clojure.jdbc](https://github.com/yogthos/clojure.jdbc) — as [jolt-lang/db](https://github.com/jolt-lang/db)'s