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/exceptions-spec.janet

@@ -35,4 +35,7 @@
   ["catch binds thrown value" "42"
    "(try (throw 42) (catch :default e e))"]
   ["rethrow preserves ex"  "\"inner\""
-   "(try (try (throw (ex-info \"inner\" {})) (catch :default e (throw e))) (catch :default e (ex-message e)))"])
+   "(try (try (throw (ex-info \"inner\" {})) (catch :default e (throw e))) (catch :default e (ex-message e)))"]
+  ["ex-data on non-ex"    "nil"       "(ex-data 42)"]
+  ["ex-cause on non-ex"   "nil"       "(ex-cause {:k 1})"]
+  ["ex-message of string" "\"hi\""    "(ex-message \"hi\")"])