feat: stub SCI host modules, fix require :as+:refer, +promoting ops/eduction

Push SCI loading much further: the giant clojure.core aggregation map in
namespaces.cljc now resolves ALL its symbols (it previously failed on dozens).

- host_stubs.clj: forward-decl no-op fns for SCI's host-layer modules
  (parser/read/load/macroexpand/resolve/proxy/reify) — Jolt supplies its own
  reader/loader, so these only need to resolve when the binding map is built.
  Stubs are non-nil fns so :refer re-interns them (a nil-valued var is dropped).
- Load Jolt's clojure.string/set/walk/edn before namespaces.cljc.
- Add real clojure.core: ==, print-str, eduction/->Eduction,
  seq-to-map-for-destructuring, split-lines, postwalk-demo/prewalk-demo,
  +'/-'/*'/inc'/dec' (Jolt doesn't overflow, so == non-quoted), proxy-super
  and friends (resolve-only, JVM-specific).

Fix a real require bug: eval-require stopped at the first option, so
[ns :as x :refer [...]] silently dropped the :refer. Now processes all options.

SCI load: clojure.core map now builds; only two forms remain (a Var-protocol
method on sci.lang.Var, and the clojure.repl namespace). conformance 218/218,
features 78/78, jank 120.

src/jolt/clojure/walk.clj

@@ -17,6 +17,16 @@
   [f form]
   (walk (partial prewalk f) identity (f form)))
 
+(defn postwalk-demo
+  "Demonstrates the behavior of postwalk by printing each form as it is walked."
+  [form]
+  (postwalk (fn [x] (print "Walked: ") (prn x) x) form))
+
+(defn prewalk-demo
+  "Demonstrates the behavior of prewalk by printing each form as it is walked."
+  [form]
+  (prewalk (fn [x] (print "Walked: ") (prn x) x) form))
+
 (defn postwalk-replace
   [smap form]
   (postwalk (fn [x] (if (contains? smap x) (get smap x) x)) form))