JVM-semantics fixes and small cleanups

- take-last / drop-last return seqs, not vectors: take-last wraps in seq; drop-last
  is the JVM (map (fn [x _] x) coll (drop n coll)) form (lazy, () when empty).
- cycle is lazy ((lazy-seq (concat coll (cycle coll)))) so it no longer counts its
  argument and terminates on a lazy/infinite input.
- fold's foldable-call catch uses :default, matching the rest of jolt-core and
  also catching a raw host condition from a folding primitive.
- alts! rejects non-channel ports with a clear error (put specs / :default are
  unsupported) instead of crashing inside ac-poll!.
- Misc: drop the unreachable second getCause clause; jolt-nth on a string raises
  'nth "index out of bounds" like the vector branch; name the inline fixpoint cap;
  bld-sh-capture rejoins lines with newlines; clarify a couple of comments.

host/chez/async.ss

@@ -152,11 +152,19 @@
           (else ac-poll-empty))))
 
 ;; (alts! [ch ...]) — take from whichever channel is ready first; returns
-;; [value channel] (value nil if that channel closed). Take-only.
+;; [value channel] (value nil if that channel closed). Take-only: every port must
+;; be a channel — put specs [ch val] and the :default option are not supported, so
+;; reject them with a clear error instead of crashing inside ac-poll!.
 ;; Polls with a 1ms backoff — no cross-channel wait-set yet.
 (define ac-1ms (make-time 'time-duration 1000000 0))
 (define (jolt-async-alts chans)
   (let ((cs (seq->list (jolt-seq chans))))
+    (for-each (lambda (c)
+                (unless (async-chan? c)
+                  (jolt-throw (jolt-ex-info
+                                "alts! supports channel ports only (put specs [ch val] and :default are not supported)"
+                                (jolt-hash-map)))))
+              cs)
     (let loop ()
       (let try ((rest cs))
         (if (null? rest)