core: Stage 3 — leaf batch 3: empty/assoc-in/update-in + interpose/take-nth to the overlay

empty, assoc-in, and update-in move to 20-coll.clj as the canonical recursive
ports; interpose and take-nth move to the lazy tier WITH their canonical
transducer arities (volatile-based), so the seed's td-interpose/td-take-nth
helpers go too. (empty lazy-seq) is () now — the kernel fn returned a bare
host table for it.

keys/vals/empty? stay put for now: they're expander-coupled — 00-syntax's
when/and/or/cond/destructure expanders call them at expansion time, which
happens during the kernel-tier compile, before any later tier exists. They
move when early defns get the staged-recompile treatment macros already have.

26 new spec rows (incl. transducer arities through sequence/into and laziness
checks against (range)). Gate green: conformance 326x3, suite >= baseline,
full jpm test.

jolt-core/clojure/core/40-lazy.clj

@@ -138,3 +138,40 @@
                (when (seq s)
                  (cons (take n s) (go (nthrest s step))))))]
      (go coll))))
+
+;; --- Phase 2 leaf batch 3 (jolt-ded): canonical lazy + transducer arities ----
+
+(defn interpose
+  ([sep]
+   (fn [rf]
+     (let [started (volatile! false)]
+       (fn
+         ([] (rf))
+         ([result] (rf result))
+         ([result input]
+          (if (deref started)
+            (let [sepr (rf result sep)]
+              (if (reduced? sepr)
+                sepr
+                (rf sepr input)))
+            (do (vreset! started true)
+                (rf result input))))))))
+  ([sep coll]
+   (drop 1 (interleave (repeat sep) coll))))
+
+(defn take-nth
+  ([n]
+   (fn [rf]
+     (let [iv (volatile! -1)]
+       (fn
+         ([] (rf))
+         ([result] (rf result))
+         ([result input]
+          (let [i (vswap! iv inc)]
+            (if (zero? (rem i n))
+              (rf result input)
+              result)))))))
+  ([n coll]
+   (lazy-seq
+     (when-let [s (seq coll)]
+       (cons (first s) (take-nth n (drop n s)))))))