Compiler research (#10)

adds self-hosted compiler is functionally:
 
- The default compile path is the portable pipeline using jolt.analyzer (Clojure) → host-neutral IR → backend.janet.
- The analyzer is itself Clojure, compiled by jolt for true self-hosting.
- bootstrap-fixpoint passes (stage1 == stage2 == stage3): rebuilding the compiler on its own output.
- clojure.core is now self-hosted in the overlay.
- Stateful forms (defmacro/ns/deftype/defmulti/require/in-ns) are interpreted by design.

test/spec/metadata-spec.janet

@@ -7,8 +7,10 @@
   ["with-meta preserves value" "true" "(= [1 2 3] (with-meta [1 2 3] {:a 1}))"]
   ["with-meta on map"     "{:doc \"x\"}" "(meta (with-meta {:k 1} {:doc \"x\"}))"]
   ["vary-meta"            "{:a 2}"     "(meta (vary-meta (with-meta [1] {:a 1}) update :a inc))"]
+  ["vary-meta extra args" "{:a 1 :b 2}" "(meta (vary-meta (with-meta [1] {:a 1}) assoc :b 2))"]
   ["meta reader ^"        "{:tag :int}" "(meta ^{:tag :int} [1 2])"]
-  ["with-meta on fn ok"   "true"       "(fn? (with-meta inc {:a 1}))"])
+  ["with-meta on fn ok"   "true"       "(fn? (with-meta inc {:a 1}))"]
+  ["with-meta nil clears" "nil"        "(meta (with-meta [1 2 3] nil))"])
 
 (defspec "metadata / type hints"
   # ^Type / ^:kw / ^"str" on a symbol attach as metadata and are otherwise inert: