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.

src/jolt/stdlib_embed.janet

@@ -30,4 +30,6 @@
   (let [acc @{}]
     (collect "src/jolt/clojure" "clojure/" acc)
     (collect "src/jolt/jolt" "jolt/" acc)
+    # Portable Clojure core (analyzer/IR/…): jolt-core/jolt/foo.clj -> jolt.foo
+    (collect "jolt-core" "" acc)
     acc))

src/jolt/types.janet

@@ -370,8 +370,9 @@
               :current-ns "user"
               :compile? compile?
               # Ordered roots searched (after the stdlib) to resolve a namespace
-              # to a .clj/.cljc file. deps.edn resolution appends dep src dirs.
-              :source-paths @["src/jolt"]
+              # to a .clj/.cljc file. jolt-core holds the portable Clojure layer
+              # (analyzer/IR/core); deps.edn resolution appends dep src dirs.
+              :source-paths @["jolt-core" "src/jolt"]
               :compiled-cache @{}
               :type-registry @{}
               :data-readers (let [dr @{}]