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

test/support/lazy-eval.janet

@@ -0,0 +1,16 @@
+# Worker: evaluate a Clojure equality check in a fresh Jolt ctx and print
+# @@RESULT true or @@RESULT false. Used by lazy-infinite-test under a wall-clock
+# deadline so infinite-seq hangs are caught as test failures.
+(use ../../src/jolt/api)
+(use ../../src/jolt/reader)
+
+(def expected (get (dyn :args) 1))
+(def actual   (get (dyn :args) 2))
+
+(when (and expected actual)
+  (def ctx (init {}))
+  (def prog (string "(= " expected " " actual ")"))
+  (def [ok val] (protect (eval-string ctx prog)))
+  (if ok
+    (printf "@@RESULT %q" val)
+    (printf "@@ERROR %q" val)))