core: move defn and defn- to the syntax tier

defn drops an optional leading docstring/attr-map then emits (def name (fn* ...)).
Single- and multi-arity both reduce to (fn* ~@body) so no arity branching is needed.
map? is true for symbol forms in Jolt, so the attr-map strip is guarded with symbol?.
defn- delegates to defn (privacy isn't enforced, as in Clojure's own defn-). Placed
before fresh-sym, which is itself a defn- now.

All five fundamental macros (fn/let/loop/defn/defn-) are now in the overlay.
conformance 228x3, fixpoint, clojure-test-suite 3930, full suite green.

jolt-core/clojure/core/00-syntax.clj

@@ -80,6 +80,21 @@
 (defmacro loop [bindings & body]
   `(loop* ~bindings ~@body))
 
+;; defn: drop an optional leading docstring and attr-map, then (def name (fn* ...)).
+;; Both single- and multi-arity reduce to (fn* ~@body) — fn* takes either a params
+;; vector + body or a sequence of ([params] body) clauses, so no arity branching is
+;; needed. (map? is true for symbol forms too, so guard the attr-map with symbol?.)
+;; Defined before fresh-sym below, which is a defn-.
+(defmacro defn [fn-name & body]
+  (let [body (if (and (seq body) (string? (first body))) (rest body) body)
+        body (if (and (seq body) (map? (first body)) (not (symbol? (first body))))
+               (rest body) body)]
+    `(def ~fn-name (fn* ~@body))))
+
+;; Jolt doesn't enforce privacy, so defn- is just defn (matching how Clojure's own
+;; defn- delegates to defn with :private metadata).
+(defmacro defn- [fn-name & body] `(defn ~fn-name ~@body))
+
 ;; A fresh jolt symbol inside a macro body (a bare (gensym) returns a Janet symbol
 ;; the destructurer rejects). This defn compiles fine: by the time a tier triggers
 ;; the analyzer build the kernel is in place (the build is gated until then).