Phase 5: true laziness — lazy transformers + deadlined harness

jolt-c09 (partial, Steps 0–2d complete).

Lazy combinator layer (Step 1):
- lazy-cons: build a LazySeq cell from val + rest-thunk (phm.janet)
- lazy-from: coerce any seqable to lazy view without forcing (core.janet)
- core-seq: check ls-first emptiness, not ls-seq realization (core.janet)

Lazy transformers (Steps 2a–2d):
Converted in Janet core.janet (lazy branches preserving transducer arities):
  drop-while, interpose, distinct, partition, partition-all,
  keep, keep-indexed, map-indexed, take-nth

Moved to Clojure overlay (jolt-core):
  interleave (20-coll.clj) — lazy multi-arity, matches Clojure
  mapcat (10-seq.clj) — transducer arity + standard (apply concat (apply map f colls))

Safety net (Step 0):
  test/support/lazy-eval.janet — subprocess worker for Clojure eval
  test/integration/lazy-infinite-test.janet — deadlined harness (os/spawn + 5s)

Multi-input map/mapcat tests (Step 2d):
  sequences-spec.janet +9 tests, verified against Clojure/CLJS references

Gates:
  lazy-infinite: 18/18 (mapcat on infinite inputs hangs — apply forces, needs Step 4)
  conformance: 229/229 ×3 (interpret/compile/self-host)
  specs: 32/32 files, 0 failures

jolt-core/clojure/core/10-seq.clj

@@ -22,3 +22,8 @@
     (if (next s)
       (recur (conj ret (first s)) (next s))
       (seq ret))))
+
+(defn mapcat
+  ([f] (comp (map f) cat))
+  ([f & colls]
+   (apply concat (apply map f colls))))