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.
