core: Phase 5 Option A — remaining lazy transformers + nth falsy-element fix

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>

test/integration/conformance-test.janet

@@ -76,6 +76,15 @@
    ["juxt fns in vec"     "[1 3]"  "((juxt first last) [1 2 3])"]
    ["last of lazy take"   "5"      "(last (take 5 (iterate inc 1)))"]
    ["next empty lazy"     "nil"    "(next (take 1 [1]))"]
+   # drop/distinct/partition/map-indexed/take-nth/interpose/keep are lazy too
+   ["drop vec is seq"     "true"   "(seq? (drop 1 [1 2 3]))"]
+   ["distinct vec is seq" "true"   "(seq? (distinct [1 1 2]))"]
+   ["map-indexed is seq"  "true"   "(seq? (map-indexed vector [1 2]))"]
+   ["partition vec lazy"  "(quote ((1 2) (3 4)))" "(partition 2 [1 2 3 4 5])"]
+   # nth over a lazy seq must not treat a false/nil element as end-of-seq
+   ["nth lazy false elem" "false"  "(nth (map identity [false 1 2]) 0)"]
+   ["nth lazy past false" "2"      "(nth (drop 1 (list false 1 2)) 1)"]
+   ["cond-> false clause" "2"      "(cond-> 1 true inc false inc)"]
 
    ### ---- HIGH: destructuring ----
    ["destr nested seq"   "[1 2 3]"   "(let [[a [b c]] [1 [2 3]]] [a b c])"]