mapcat in 10-seq.clj now uses a defn- mapcat-step helper that walks
lazy map results element-by-element (cons + lazy-seq), with explicit
arities for 1/2/3/4+ colls. The 4+-coll arity still uses apply to
spread colls to map, but apply no longer forces the inner map result.
Previously (apply concat (apply map f colls)) forced the lazy map
result through core-apply realize-for-iteration, which hung on
infinite inputs. The new step-based impl walks one element at a time
via first/rest/seq primitives.
Re-enabled deferred mapcat infinite-input harness case. 19/19 pass.
Gates: lazy-infinite 19/19, conformance 229/229 interpret+self-host
(compile 228/229 — 1 pre-existing protocol error), specs 32/32.
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