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)

host/chez/build.ss

@@ -27,9 +27,12 @@
       (let ((l (get-line in)))
         (if (eof-object? l)
             (begin (close-port in)
-                   (let ((s (apply string-append (reverse acc))))
-                     ;; trim a trailing newline-equivalent (we joined without them)
-                     s))
+                   ;; rejoin with newlines (get-line stripped them). Callers use
+                   ;; single-line output; this just avoids silently concatenating
+                   ;; two lines into one corrupt token if a command emits more.
+                   (let ((ls (reverse acc)))
+                     (if (null? ls) ""
+                         (fold-left (lambda (s x) (string-append s "\n" x)) (car ls) (cdr ls)))))
             (loop (cons l acc)))))))
 
 (define (bld-system cmd)

host/chez/collections.ss

@@ -243,7 +243,8 @@
        (cond ((pvec? coll) (let ((v (pvec-v coll)))
                              (if (and (fx>=? i 0) (fx<? i (vector-length v))) (vector-ref v i)
                                  (error 'nth "index out of bounds"))))
-             ((string? coll) (string-ref coll i))
+             ((string? coll) (if (and (fx>=? i 0) (fx<? i (string-length coll))) (string-ref coll i)
+                                 (error 'nth "index out of bounds")))
              ((or (cseq? coll) (empty-list-t? coll)) (seq-nth coll i #f jolt-nil))
              (else (error 'nth "unsupported collection")))))
     ((coll i d)

host/chez/records.ss

@@ -174,7 +174,9 @@
                  (keyword #f "name") (jolt-symbol jolt-nil name-str)
                  (keyword #f "methods") methods))
 
-;; register-protocol-methods!: a no-op for Chez dispatch.
+;; register-protocol-methods!: intentional no-op. Chez dispatches a protocol method
+;; by the receiver's type tag at call time, so there is no method table to register;
+;; this binding exists only because defprotocol-emitted code calls it.
 (define (register-protocol-methods! proto-name method-names) jolt-nil)
 
 ;; register-method: extend-type/extend register an impl. Host type names keep a
@@ -329,7 +331,7 @@
              ((string=? method-name "toString") (condition->message-string obj))
              ((string=? method-name "getCause") jolt-nil)
              ;; java.sql.SQLException chaining — jolt errors don't chain (nil).
-             ((or (string=? method-name "getNextException") (string=? method-name "getCause")) jolt-nil)
+             ((string=? method-name "getNextException") jolt-nil)
              ((string=? method-name "getStackTrace") (jolt-vector))
              ((string=? method-name "printStackTrace") jolt-nil)
              (else (error #f (string-append "No method " method-name " on Throwable")))))