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).

test/spec/strings-spec.janet

@@ -53,3 +53,12 @@
   ["hyphens to underscores" "\"a_b_c\"" "(namespace-munge \"a-b-c\")"]
   ["from a symbol"          "\"foo_bar\"" "(namespace-munge (quote foo-bar))"]
   ["no hyphens unchanged"   "\"ok\""    "(namespace-munge \"ok\")"])
+
+# (get s i) indexes a string and returns the char, like Clojure (nth already
+# did; get did not — reitit's path parser relies on it).
+(defspec "strings / get indexes a string"
+  ["get returns the char"  "true"  "(= (get \"a:b\" 1) \\:)"]
+  ["get first char"        "\\a"    "(get \"abc\" 0)"]
+  ["get out of range nil"  "nil"   "(get \"abc\" 9)"]
+  ["get negative nil"      "nil"   "(get \"abc\" -1)"]
+  ["get default honored"   ":none" "(get \"abc\" 9 :none)"])