Converts the rest of the lazy transformers to return a LazySeq even over a
concrete collection (dropping the eager "preserve representation" branch, which
returned a vector over vector input): drop, distinct, partition, partition-all,
map-indexed, keep, keep-indexed, take-nth, interpose. Each collapses to its
existing lazy branch run over (lazy-from coll); transducer arities unchanged.
Fixes a latent nth bug exposed by this: core-nth's lazy branch walked with
(ls-first cur) truthiness as the end-of-seq test, so a legitimate false/nil
element was mistaken for the end — (nth (map identity [false 1 2]) 0) threw
instead of returning false, and cond-> (whose macro nths over a now-lazy
(drop 2 clauses) containing the boolean clause tests) failed. nth now walks
with seq-done? and reads via core-first.
Gate: conformance 246x3 (+7 cases), lazy-infinite 18/18, fixpoint, self-host,
all specs+unit green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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