Chez Phase 2 (inc H): volatiles + sequence/transduce

sequence and transduce were seed natives nil on the prelude; the stateful
transducer arities (take-nth/map-indexed/partition-by, overlay) drive a
volatile via volatile!/vswap!/vreset!/deref, also unshimmed — so the whole
(sequence xform coll) / (transduce xform f coll) surface crashed.

natives-xform.ss: native volatiles (a jvol record + volatile!/vreset!/
vswap!/volatile? + a deref arm); transduce/sequence built on the existing
into-xform / reduce-seq. The overlay vreset!/vswap!/volatile? drive a
volatile through ref-put!/:jolt/type (tagged-table only), so they're
overridden natively in post-prelude.ss.

driver.janet: drain each chez pipe to EOF instead of a single ev/read. A
program with a stdout side effect (newline) flushes in two writes and a
lone ev/read sometimes caught only the first chunk, dropping the trailing
value — an intermittent gate divergence. Now reads until the pipe closes.

Prelude parity 1886 -> 1898, 0 new divergences. Floor raised to 1898.

test/chez/run-corpus-prelude.janet

@@ -176,8 +176,12 @@
 # referential lazy-cat (fib) stays productive. Unblocks repeat/iterate/cycle/
 # dedupe/take-nth/keep/interpose/reductions/map-indexed/distinct/interleave/
 # tree-seq->flatten/partition-all/lazy-cat) 1886.
+# Phase 2 inc H (jolt-xjx6: native volatiles (jvol) + sequence/transduce over the
+# existing into-xform/reduce-seq — unblocks (sequence xform coll), (transduce
+# xform f coll), and the stateful transducer xforms take-nth/map-indexed/
+# partition-by that drive a volatile) 1898.
 # Strided runs scale down.
-(def base-floor (scan-number (or (os/getenv "JOLT_CHEZ_PRELUDE_FLOOR") "1886")))
+(def base-floor (scan-number (or (os/getenv "JOLT_CHEZ_PRELUDE_FLOOR") "1898")))
 (def floor (if (os/getenv "JOLT_CORPUS_LIMIT") 0 base-floor))
 (when (or (> (length diverged) 0) (< pass floor))
   (printf "REGRESSION: pass %d < floor %d or %d new divergence(s)" pass floor (length diverged)))