Step 4: evaluator eager assumptions — lazy rest + lazy mapcat

Fix & rest destructuring (evaluator.janet): when the original
value is a lazy-seq, derive & rest by walking ls-rest vi steps
instead of slicing the eagerly-realized array from d-realize.

Rewrite core-mapcat as a lazy state machine (core.janet): step
through input colls one element at a time, call f on each tuple,
then yield from f's result collection. No apply-forcing —
all input colls are walked lazily via lazy-from + seq-done?.

Add mapcat to core-renames (compiler.janet) and remove from
Clojure overlay (10-seq.clj) — compile mode now emits direct
calls to the lazy core-mapcat, fixing the pre-existing
protocol-on-record compile error.

Tests: re-enabled mapcat infinite harness case, added 2 & rest
laziness cases. 21/21 lazy-infinite, 229x3 conformance, 32/32 specs.

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

@@ -22,8 +22,3 @@
     (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))))