self-host: scaffold portable jolt-core + host/core architecture

Adds doc/self-hosting-architecture.md: the portability design (portable front
end + host back end + host runtime, seam = a minimal host protocol + the IR),
grounded in the CLJS/Clojure-in-Clojure prior art. Decisions locked: minimal host
protocol as the seam; physical split with a jolt-core/ source root.

Adds jolt-core/ as a portable Clojure source root (dev path + embedded into the
binary alongside the rest of the stdlib) and jolt-core/jolt/ir.clj — host-neutral
IR node constructors (vars referenced by name, no host values embedded). Verified
it loads and runs under both interpreter and compiler.

jolt-core/jolt/ir.clj

@@ -0,0 +1,42 @@
+(ns jolt.ir
+  "Host-neutral intermediate representation for the Jolt compiler.
+
+  The analyzer (jolt.analyzer) produces IR; a host back end consumes it. IR nodes
+  are plain maps tagged with :op — no host values embedded. Globals reference vars
+  by name (:ns/:name), never by a host var cell, so the IR is portable and
+  AOT-safe. This namespace is pure Clojure (portable jolt-core): it depends on
+  nothing host-specific.")
+
+;; Node constructors. Kept as data so any back end can pattern-match on :op.
+
+(defn const [v] {:op :const :val v})
+
+(defn local [name] {:op :local :name name})
+
+;; A global var reference, by name. The back end resolves it to a host var.
+(defn var-ref [ns name] {:op :var :ns ns :name name})
+
+;; A runtime primitive (cons, +, get, apply, …) the back end maps to the host RT.
+(defn rt [name] {:op :rt :name name})
+
+(defn if-node [test then else] {:op :if :test test :then then :else else})
+
+(defn do-node [statements ret] {:op :do :statements statements :ret ret})
+
+(defn invoke [f args] {:op :invoke :fn f :args args})
+
+(defn def-node [ns name init] {:op :def :ns ns :name name :init init})
+
+(defn let-node [bindings body] {:op :let :bindings bindings :body body})
+
+;; A fn is one or more arities. Each arity: {:params [..] :rest name|nil :body ir}.
+(defn fn-node [name arities] {:op :fn :name name :arities arities})
+
+(defn vector-node [items] {:op :vector :items items})
+(defn map-node [pairs] {:op :map :pairs pairs})
+(defn set-node [items] {:op :set :items items})
+
+(defn quote-node [form] {:op :quote :form form})
+(defn throw-node [expr] {:op :throw :expr expr})
+
+(defn op [node] (:op node))